BOOK REVIEWS

Optical Instrhments. By EARLE B. BROWN. pp. XII , 567-[-211 Figs., 21~X 13½ cm. Chemical Publishing Co., Inc., Brooklyn, New York. Price $10.00.

The subject matter of this book is primarily geometric optics and its applications. Part I, entitled "Principles of Geometrical Optics" (]74 pages), includes the most im- portant material of this subject in a somewhat condensed form, plus some new material on image inversion and totally reflecting prisms. Pa r t II, entitled "Description, Opera- tion, and Theory of Optical Instruments" (197 pages) deals almost exclusively with their description and operation. Chapter X X I I (31 pages) "The Spectroscope" describes the general aspects of this instrument and the ruled diffraction grating. Chapter XXV (21 pages) "Military Instruments," and Chapter XXVI (15 pages) "l~ange Finders" contain some new material, limited for reasons of security. The author states that, "The instru- ments and principles which we shall discuss are known to and in use by the armies and navies of all the world at the present time." Part I II , entitled "Construction and Main- tenance of Optical Instruments" (62 pages) contains a good deal of useful information again in somewhat condensed form. Part IV, entitled "Supplementary Topics" (23 pages), consists chiefly of "Notes on Physical Optics." In addition, there is included an appendix of "Mathematical Proofs" and another entitled "Glossary of Optical Instru- ment Terms."

In the "Introduction" it is stated that., "The author has attempted to present the broad field of optical instruments as a related whole and to prepare a volume which will be of value and interest to students and teachers in schools and industrial plants, person- nel of military and civil establishments, workers and repairmen whose occupation has brought them into contact with various types of optical instruments which it would be to their interest and advantage to understand." I t would have been surprising if the author had succeeded in filling so large an order. I t appears to this reviewer that the book would serve as a good introduction to geometrical optics for the last four categories enumerated above, but it is not sufficiently comprehensive or detailed in its treatment to serve as a fcxt for students or teachers. In order thoroughly to understand any of the instruments discussed it would be necessary to seek further details in some other source. In this connection it should be mentioned that no refcrenc~es or bibliography are in- eluded.

The author also states in the "Introduction" that, "This book has been written in an effort to fulfill the need for a volume which will make a reasonably thorough coverage of the general field of optical instruments. Present volumes are either restricted to a thorough coverage of one instrument or of a small group of similar instruments, or to a theoretical discussion of optical principles." With the exception of the spectroscope and diffraction grating, this book deals almost exclusively with the general field of geometric optics. Comparison with a well known text published in 1932 reveals that the present volume makes no mention of photometers, spectrophotometers, refractometers, densitom- eters, pyrometers, radiometers and the like. The Michelson interferometer is dis- cussed on one page hut no other interferometers are mentioned. Only one form of polariscope, the polarimeter, is described. A few instruments which do not appear in the earlier work are described in the present volume, namely: the Schmidt camera, the sextant, theodolite, opaque projector, cystoscope and some of the prisms.

I t seems surprising that a book of 500 pages of text does not contain more necessary details. This is in part due to the large readable type and the somewhat small sized

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pages which contain about 320 words each (compared to the above mentioned text of over 600 pages containing about 400 words per page). In addition there is a considerable discussion of useful mechanical details which have, however, crowded out equally neces- sary optical details. Also, many of the 230 figures, particularly the line drawings, are larger and occupy more space than seems necessary. Some of the drawings leave much to be desired, particularly Figure 19, which illustrates total internal reflection under a condition where i t would not occur; Figure 55 "Structure of the Human Eye" which is badly out of proportion; Figure 107 "Photoelectric Exposure Meter" which is so blacked out t ha t its details are indistinguishable; Figure 177 "The Aiming Circle" which is so hasti ly drawn tha t its details are unintelligible, and Figure 208 "Interference of Light from Two Sources" which is improperly drawn. I t is unfor tunate t ha t there are only two photographs, both of which are good.

Most of the principles of geometrical optics are well presented, the derivations of the lens equations being, on the whole, readily understandable. However, i t is confusing to have the basic equations for paraxial rays first derived for the case of reflection a t a convex spherical mirror when both the object and image are vir tual (Figure 22).

Chapter X "Tota l Reflecting Prisms" presents the subject of image inversion which, as the au thor says in Chapter II , is "quite confusing, and some a t t empt to clarify i t will be made in Chapter X . " The a t t empt was unsuccessful, a t least for this reviewer. The discussion is based on the appearance of one's own reflection in a plane mirror as follows (page 11) : "To an observer looking at us from a point behind the mirror, our left would be at his right, and our r ight a t his left. As we see ourselves in the mirror, however, our left appears a t our left and our right at our right. In other words, our image in the mirror, as seen by us, is inverted from left to right in a horizontal plane, or around a vertical axis." I t might seem tha t the criterion of like opposite like is the criterion of lack of inversion rather than inversion, since as we see ourselves in the mirror, our head is opposite our head and our feet opposite our feet, which condition the author does not consider an inversion. The real image produced by a convex lens is considered to be inverted about two axes, since its top is opposite the bot tom of the obiect and its right opposite the left of the object. The axis of inversion defined by the author is determined by the inversion of the observer behind the mirror who has a subjective preference to turn himself about a vertical axis. If he had stood on his head to observe us, we would then have concluded t ha t the mirror inverted about a horizontM axis. We .would have reached the same conclusion if we had observed ourselves in a mirror while lying on our side.

On page 90 the author says, " I f the effect of a prism is to deviate the line of sight through 180 °, this deviation is exactly the same as an inversion about the axis of devi- at, ion. This phenomenon effectively perrhits the observer to get behind the object and view it from tha t vantage point. The effect can easily be seen by holding a t ransparent picture between oneself and a plane mirror. The direct view of the picture and its image in the mirror are identical, the result of the summation of the inversion produced by the mirror and the 180 ° deviation of the line of sight. This is not the same thing as using a mirror to view something behind oneself, in which case the image is inverted as compared with the observer's normal view, which would require t ha t he turn around."

There is not space here to a t t empt to clarify this problem. I t is suggested t ha t a care- ful distinction should be made between rotat ion of an image in space and inversion of the aspect of the image. For example :(1) Normal incidence in a mirror produces no rotat ion in space but does produce 180 ° deviation in the line of sight. The axis of devia- tion is indeterminate as far as the mirror is concerned, bu t depends on the orientation of the object and our subjective preference for viewing it. (2) Grazing incidence produces 180 ° rotat ion of the image in the plane of vision, bu t no deviation of the line of sight. (3) Two mirrors a t r ight angles to each other rotate the image through 180 ° and also

BOOK REVIEWS 475

deviate the line of sight through 180 ° . The aspect of the image is then the same as that of the object when viewed from behind the mirrors. But in describing the roof prism, the equivalent of two mirrors at right angles, the author says on page 103: " In this case the planes of incidence for the two reflecting surfaces are mutually perpendicular and inversion in two planes results." Observation shows that the image is not inverted ac- cording to the author's previous definition of inversion, and when viewing at right angles to the line of intersection of the mirrors, the planes of incidence for the two surfaces are parallel and coincident!

Perhaps the difficulty lies in the definition of inversion. The author's definition is certainly not the same as that applied to the doubly inverted image produced by a con- vex lens, which is rotated in space through 180 ° about two mutually perpendicular axes, and is generally c6nsidered to be inverted regardless of the direction in which it is viewed.

The subject of resolving power is treated very briefly and lens aberrations are dis- cussed clearly but hardly in sufficient detail.

Several errors were noted in Chapter X X X I I I , "l~otes on Physical Optics." The following two sentences on page 455 are apparently garbled: "Either wave=length or frequency may be used as a measure in amplitude. When two wave trains of the same wave=length and low frequency being red and low in energy; that of short wave-length and high frequency being voilet and of high energy." The two words "and amplitude" should be added after "the same wave-length" in the following sentence (page 455). "When two wave trains of the same wave-length are one-half a wave-length (180 °) out of phase, the resultant motion of the particle is zero,-- ."

On page 458 the equation for the spacing of the interference fringes produced by two coherent sources is given incorrectly. On page 470 the resultant according to Huygen's principle of a plane wave at a point is incorrectly derived.

As suggested above this book provides the technician with a useful introduction to geometric optics and the methods of construction and operation of instruments based thereon. However, it does not appear that much has been included that cannot already be found in one volume.

DAVID SINCLAIR, Johns=Manville Research Laboratory

Chemical Process Principles, Part I: Material and Energy Balances. OLAF A. HOUGEN AND KENNETH M. WATSON. (Second Printing, 1944) 436 pages. John Wiley and Sons, Inc. Price $4.50.

This book represents an expansion of the first part of the authors' older text, .INDUSTRIAL CHEMICAL CALCULATIONS. The aim of the present volume, as indicated by the title, is to introduce the student to the technique involved in setting up material and energy balances for complex chemical processes. In the course of arriving at this objective detailed treatments of stoichiometry, gases and gas mixtures, latent and sensible heat effects and thermochemistry are included. Solution effects and the use of phase diagrams are also covered. A notable feature of this, as of previou~ editions and printings of this work, is the inclusion of an extensive discussion of methods for estimating missing data. These methods have been brought up to date and represent an extremely useful tool for student and practicing engineer alike.

The material is clearly presented in a logical manner. Every effort has been made to make this volume useful to and usable by the undergraduate engineering student. Occasionally the authors' efforts at simplification have led to inaccuracies of statement, notably in the discussion of adiabatic saturation temperature and wet bulb temperature on page 104. The identity of these values under certain fixed conditions for water-air systems has been carried over to other systems for which the two temperatures can,