i8o 



SCIENCE. 



BOOKS RECEIVED. 



Sight : An Exposition of the Principles of Monocular and 

 Binocular Vision, by Joseph Le Conte, LL.D., Pro- 

 fessor of Geology and Natural History in the Univers- 

 ity of California. — D. Appleton & Co. New York, 

 1881. 



This is another of those books specially prepared to 

 bring wiihin the reach of the general reader, some of the 

 most interesting but complicated problems of science. 



In the present case Professor Le Conte has attempted 

 the double task of writing a work for the general public, 

 which should also be found profitable reading for even 

 the most advanced specialist. This would appear to be 

 an embarrassing operation even with a popular subject, but 

 with one so purely technical as optics the difficulties of the 

 author must have been greatly increased. 



To accomplish this programme with success, Professor 

 Le Conte has divided his subject into two parts ; the first 

 treating, in an elementary manner, the anatomy and con- 

 struction of the human eye, and the various theories in- 

 volved in vision ; and also a second part addressed to the 

 specialist, in which some of the disputed points in Binoc- 

 ular vision are treated in detail. 



Professor Le Conte, in speaking of the wonderful 

 mechanism of the human eye, describes it as a master- 

 piece of Nature, whereas it is well known that many aj- 

 thors have been far less enthusiastic in its praise. 



To show how differently two authors may treat the same 

 subject even from the same point of view, let us com- 

 pare what Professor Le Conte and Dr. H. Newell Mar- 

 tin says, as to the comparison of the human eye with a 

 microscope objective, for curiously enough they both se- 

 lect the same object for this purpose, the former in the 

 work now under review, and the latter in " The Human 

 Body," a book recently received in our office, a notice 

 of which is in preparation. 



Professor Le Conte remarks, * " We see, then, that the 

 mode of adjustment of the eye is somewhat like that of 

 the Microscope." " Like the Microscope, but how in- 

 finitely superior." 



Whereas, Dr. Martin observes : " The eye, though it 

 answers admirably as a physiological instrument, is by 

 no means as perfect optically; not nearly so good, for ex- 

 ample, as a good Microscope objective."! Again Pro- 

 fessor Le Conte, while indeed speaking of " defects of the 

 eye as an instrument," refers such imperfections to a 

 condition of disease or malformation, or at least to eyes 

 which are not " normal or perfect," and specially men- 

 tions Myopy or Brachymetropy, Presbyopy, Hypermetropy 

 and Astigmatism. Whereas, Dr. Martin points out de- 

 fects of the human eye, which appear to be the normal 

 condition of human vision, and exist "even in the best of 

 eyes" — such as the " reflecting surfaces of our eyes not 

 even being truly spherical, especially in the case of the 

 cornea," and that " few persons are able to see equally 

 clearly, at once, two lines crossing one another at right 

 angles," this defect, when existing in a marked degree, 

 causing serious trouble, amounting to disease, and known 

 as "astigmatism." 



In regard to the first part of the work, we may state 

 that rarely has the subject been before treated in a man- 

 ner so likely to realize in the mind of the student all that 

 it is necessary to know in regard to the vision of man. 

 The author's descriptions are so clear that to miscon- 

 strue them is impossible. Excellent illustrations are 

 given, the majority being diagrams prepared by Professor 

 Le Conte. 



In regard to the more technical parts of the work relat- 

 ing to Binocular Vision, we find a difficulty in giving a 

 resumd without diagrams, which it is not possible to 

 present with this notice, but we trust we have aroused suf- 



* Sight, &c, page .15. 



t The Human Body, page 502. 



ficient interest in this book to induce those studying 

 the question to make direct use of Professor Le Conte's 

 work, where the whole subject is explained. 



Professor Le Conte's own views, however, on Binocular 

 vision may be given, which are expressed in the following 

 words : 



" All objects or points of objects, either beyond or nearer 

 than the point of sight, are doubled but differently . 

 The former homonymously, the latter heteronymously. 

 The double images in the former case are united by less 

 convergence, in the latter case by greater convergence of 

 the optic axes. Now the observer knows instinctively 

 and without trial, in any case of double images, whether 

 they will be united by greater or less optic convergence, 

 and therefore never makes a mistake, or attempts to unite 

 by making a wrong movement of the optic axes. In 

 other words, the eye (or the mind) instinctively distin- 

 guishes homonymous from heteronymous images, refer- 

 ring the former to objects, successive combination of 

 the different parts of -the object or scene, or pictures, as 

 maintained by Briicke." \ 



Professor Le Conte claims that this work " meets a 

 real want, and fills a real gap in scientific literature;" in 

 this assertion we heartily concur. 



CORRESPONDENCE. 



L The Editor does not hold himself responsible for opinions expressed 

 by his correspondents. No notice is taken 0/ anonymous communi- 

 cations.] 



A PROBLEM IN OPTICS. 



To the Editor of "SCIENCE:" 



Will some of your correspondents answer the follow- 

 ing questions, not from any tables given in books, but by 

 original computation or observation. Given a plate of 

 crown glass, index of refraction 1.525, with parallel sur- 

 faces, a ray of light incident on the under-surface 38 from 

 the normal. 



What will be the direction of the ray in the glass ? 

 What will be its direction of emergence from the upper 

 surface into air? What its direction into water? What 

 percentage of the light will be lost by reflection from the 

 under surface of the plate ? What will be lost from the 

 upper surface ? 



If the light is incident 45 from the normal, what is the 

 answer to each of the above questions ? 



As each color of the spectrum has a different degree 

 of refraction the medium ray should as usual be taken 

 for computation. 



The thickness of the plate cannot affect the refrac- 

 tion in the glass, only the point where the ray reaches 

 the upper surface. Carl Reddots. 



Ozone is absorbed in a solution of arsenious acid to 

 which a little neutral potassium iodide has been added. 

 The excess of arsenious acid is estimated with standard 

 iodine solution. 1. Ozone is a constituent of the higher atmos- 

 phere to a much larger extent than near the earth's surface. 

 2. Ozone is destroyed by contact with the gases and organic 

 matter in a moist atmosphere, as near the earth's surface, 

 and that the collection of ozone from the air is attended by 

 the destruction of it to a large extent. 3. The absorptive 

 power of the ozone in the air is quite sufficient to account 

 for the limitation of the solar spectrum. 4. The blue tint 

 of the atmosphere is largely due to ozone. — W. N. Hart- 

 ley. 



ERRATA. 



How to obtain the Brain of the Cat, (Wilder). — Correction : 

 Page 158, second column, line 7, "grains," should be 

 "grams;" page 159, near middle of 2nd column, " suc- 

 cessily," should be "successively;" page i6r, the number 

 of Flower's paper is 3. 



% Page 151. 



