NATURE 



217 



THURSDAY, JULY 9, 1903. 



RECENT WORKS ON OPTICS. 



Manual of Advanced Optics. By C. Riborg Mann, 

 Assistant Professor of Physics in the University of 

 Chicago. Pp. 196. (Chicago : Scott, Forseman 

 and Co., 1902.) 



Practical Exercises in Light : being a Laboratory 

 Course for Schools of Science and Colleges. By 

 R. S. Clay, B.A., D.Sc. Pp. vi+187. (London: 

 Macmillan and Co., Ltd., 1902.) Price 2S. 6d. 



Elementary Ophthalmic Optics. By Freeland Fergus, 

 M.D., F.R.S.E., Surgeon to the Glasgow Eye In- 

 firmary. Pp. viii + 107. (London : Blackie and 

 Son, 1903.) Price 3s. 6d. net. 



Geometrical Optics : an Elementary Treatise upon the 

 Theory, and its Practical Application to the more 

 Exact Measurements of Optical Properties. By 

 Thomas H. Blakesley, M.A. Pp. viii + 120. 

 (London : Whittaker and Co., 1903.) Price 25. 6d. 



Das Stereoskop. Seine anwendung in den technischen 

 Wissenschaften. Uber Entstehung und Konstruk- 

 tion Stereoskopischer Bilder. Von Wilhelm Man- 

 chot, Architekt und Professor am Stadel'schen 

 Kunstinstitut zu Frankfurt a.M. Pp. vi (3 blank) + 

 68. (Leipzig: Vcit and Co., 1903.) 



MR. MANN'S book contains an account of the three 

 months' experimental course on optics pursued 

 by the senior students at the University of Chicago. 

 The name Chicago, uttered on this side of the Atlantic, 

 suggests many different things to different persons ; 

 to physicists it cannot but bring to mind the name of 

 Prof. Michelson, to whom we are indebted for some of 

 the most valuable and ingenious optical investigations 

 that the last century brought forth. An experimental 

 course, developed according to the ideas of Prof. 

 Michelson, could scarcely be other than original and 

 stimulating; the course before us, in addition, is 

 systematically developed, the descriptions are clear and 

 concise, and the illustrations, though few, are well 

 calculated to serve the purpose for which they were 

 intended. Each chapter commences with a brief theo- 

 retical investigation, wherein the aim is to concentrate 

 attention on the physical, as distinguished from the 

 purely mathematical, aspect of the subject; following 

 this is a description of the experiments, and the 

 manipulation of the necessary apparatus. Room is 

 left for the student to develop a certain amount of 

 originality in his methods, and thus avoid reduction 

 to the state of a mere mechanical copyist. Numerous 

 references are given to original memoirs, which should 

 prove very useful to advanced students. Besides the 

 experiments usually found in books on optics, descrip- 

 tions are given of the method of determining the re- 

 solving power of telescopes, spectroscopes, and 

 gratings. It is interesting to find Prof. Michelson 's 

 classic researches on the resolution of spectral lines by 

 means of the interferometer included in a course for 

 students. In fine, no one on glancing through this 



NO. 1758, VOL. 68] 



book would hesitate to endorse the concluding words 

 of Prof. Michelson's introductory note: — 



" Those who desire to enter into optical investi- 

 gations cannot get a better foundation for future work 

 than by studying the optical theories here presented, 

 and performing the experiments described." 



Teachers have long felt the want of an inexpensive 

 book on practical light, suitable for students who are 

 commencing the study of the subject ; and to these Dr. 

 Clay's little book may be confidently recommended. 

 As stated in the preface, it forms the elementary portion 

 of a " Treatise on Practical Light," now in preparation 

 by the author. It is by no means an easy task to 

 arrange a series of elementary experiments on light, 

 which shall be sufficiently varied to prevent the interest 

 of the student from flagging, while of sufficient scope 

 and completeness to give the student a firm grasp of 

 the elementary principles of the subject. The author 

 is to be congratulated on his success in both the above 

 respects. The ordinary laws of reflection, refraction, 

 and dispersion are illustrated by the aid of simple ex- 

 periments, which can be performed without the aid of 

 expensive appliances; indeed, the spectrometer is the 

 only piece of elaborate apparatus required for the course 

 described. In addition, numerous practical exercises 

 are appended at the ends of the chapters. Attention is 

 directed to the observation of caustics, and the 

 principle of formation of the rainbow. The optical 

 bench described by the author is to be commended for 

 its simplicity and ef!icacy. Perhaps the most novel part 

 of the course consists in a number of experiments on the 

 optical properties of the eye, and others on diffraction 

 and interference. We do not remember to have pre- 

 viously seen a description of the method of producing 

 Lloyd's single mirror fringes by the aid of a prism and 

 spectrometer. Points to which objections can be raised 

 are neither numerous nor important. On p. 86, it is 

 stated that a telescope focused for infinity and directed 

 towards the sun gives rise to a parallel bundle of emer- 

 gent rays. This is scarcely correct; the rays from any 

 particular point of the sun will emerge parallel to each 

 other, but the total emergent light consists of a diverg- 

 ing bundle of pencils, each consisting of parallel rays. 

 On p. 127, it is stated that the fovea centralis contains 

 rods only and no cones, while the reverse is actually 

 the case. The account given on p. 132 of the mechan- 

 ism of accommodation could bear revision ; modern re- 

 search indicates that the increased curvature of the 

 anterior surface of the crystalline lens is produced by 

 an increase of tension in the anterior capsule layer, 

 and not by its relaxation, as was supposed by Helm- 

 holtz. 



The opththalmic surgeon has to deal with the eye, 

 not alone as a delicate organ of the human body, sub- 

 ject, like other organs, to disease ; but also, in many 

 cases, as a defective optical instrument. Hence a 

 knowledge of optics is as necessary to him as an 

 acquaintance with the science of electricity is neces- 

 sary to the electrical engineer. Dr. Fergus's book 

 has been written for medical students, as an 

 elementary introduction to the science of geometrical 



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