7o 



NA TURE 



[May 1 8, i! 



with the optic vertex, and to the distance of this the beat 

 adjustment is adapted. 



Let o = optic angle, varied by mean-; of the stereoscope. 

 o' = optic angle of normal vision for given distance. 

 D =distance of optic vertex from each eye, determined 

 by the formula, D = A i co-ec Jo. 

 ( D'=distance of radial point measured in the direction 

 from which the reflected ray enters the eye. It is 

 hence the distance of the vir ual image in normal 

 vision. 

 ,, A = dhtance of point of sight from binocular eye. 

 Under the conditions given above we have — 

 a = a, and A = D=D'. Assume D' =50 cm., then a' =7° 20'. 

 If now we make a = 37 Q 20', we have D = locm. But to 

 secure distinct vision, the focal adjustment must be adapted to 

 D', and therefore dissociated from the axial adjustment. This 

 to some extent antagonises the effect of tension of the internal 

 rectus muscles, and this antagonism is increased by the fact that 

 the vi-ual angle remains constant. The combined effect is that 

 A>Dbut A<D'. The apparent size of the image is dimin- 

 ished in 1 he ratio of A to D'. The effect of increasing the 

 optic an,de i; hence to make the image appear nearer, smaller, 

 and less deep in proportion to its area, but more distant never- 

 theless than the new optic vertex. 



If now we makea=-5°, we have D=-73'4cm., but the 

 relaxation of the internal rectus and contraction of the external 

 rectus mu-cles causes the image to appear to recede in a positive 

 direction. This illusion is opposed by the constancy of the 

 visual an^le, and the ciliary effort to keep the focal adjustment 

 adapted to D'. The result is that A>D', and the apparent size 

 of tbe image is enlarged in the same ratio, while its depth is 

 increased still more. The effect of making the optic angle 

 negative is hence to cause the image to appear farther, larger, 

 and deeper in proportion to its area. 



If in the discussion just given we make o the angle between a 

 pair of camera axes, and D the distance of its vertex, while 1 is 

 the distance between the two lenses, the formula is readily 

 applicable, but a can have only positive values. The optic angle 

 for the observer while using the stereoscope is not necessarily, or 

 even generally, the same as that between the camera axes when 

 the picture was taken. Apparent distance in the stereoscope is 

 thus not determined by the intersection of the observer's visual 

 lines, and no mathematical formula can be made to apply to the 

 interpretation of muscular tension in the muscles of the eyes. 

 The error into which Wheatstone fell, and which was repeated 

 and emphasised by Brewster, consists in the application of 

 geometry where physiological conditions are such as to destroy 

 the value of all geometric constructions. Unfortunately this 

 error is still repeated in most of our text-books of physics, 

 wherever diagrams are employed to explain the theory of the 

 stereoscope. W. Le Conte Stevens 



New York 



SCIENTIFIC SERIALS 



The Quarterly Journal of Microscopical Science for April, 

 1882, contains — Pringsheim's researches on chlorophyll, trans- 

 lated and condensed by Professor Bayley Balfour (with plates 

 8 and 9). — Dr. D. H. Scott, on the development of arti- 

 culated laticiferous vessels (plate 10). In the plants inves- 

 tigated, the vessels arose from rows of cells, of which the cross 

 walls, and where two were in contact, the side walls in part 

 became gradually absorbed. This took place very early ; when 

 not in contact, connection took place by means of cross rows of 

 cells, which underwent fusion, or by inoculating outgrowths, 

 before absorption ; such cells showed the probable pre ence of 

 latex. — Dr. E. Klein, on the lymphatic system and the minute 

 structure of the salivary glands and pancreas (plates 11 and 12). 

 — rrof. F. M. Balfour and F. Deighton, a renewed study of 

 the germinal layers of the chick (plates 13-15). — Isao J. Iijima, 

 on the origin and growth of the eggs and egg-strings in Nephelis, 

 with some observations on the "spiral asters " (plates 16-19). — 

 Dr. A. A. Hubrecht, a contribution to the morphology of the 

 Amphineura. — Prof. E. Kay Lankester, on the chlorophyll- 

 corpuscles and amyloid deposits of Spongilla and Hydra (plate 

 20). These forms are not of the nature of parasitic bodies, 

 but they correspond in structure with the chlorophyll bodies in 

 plants. 



Journal of the Royal Microscopical Society for April, 1882, 

 contains the President's address, by Prof. B. Martin Duncan. — 



On mounting objects in phosphorus, and in a solution of biniodide 

 of mercury and iodi le of potassium, by J. W. Stephenson. — On 

 the threads of spider webs, by Dr. J. Anthony. — With the usual 

 most u-eful summary of current researches relating to geology 

 and botany, and the Proceedings of the Society. 



Journal of Anatomy and Physiology, Normal ami Patho- 

 logical, vol. xvi. Part 3, April, 1882, contains — Dr. A. M. 

 Marshall, the segmental value of the cranial nerves (pi. 10). — 

 Dr. G. E. Dobson, the anatomy of Microgale longicauda, with 

 remarks on the homologies of the long flexors of the toes in 

 mammalia. — Dr. T. P. A. Stuart, the curled hair and curled 

 hair follicles of the Negro. — Dr. G. Sims Woodhead, some of 

 the pathological conditions of the medulla oblongata, in a case 

 of locomotor ataxia (pi. 11). — Dr. M. Hay, on the action of 

 saline cathartics. — W. J. Walsham, abnormal origin aid distri- 

 bution of the upper seven right intercostal arteries, with remarks. 

 — Dr. W. Stirling, on the digestion of blood by the common 

 leech, and on the formation haemoglobin crystals (pi. 12). — 

 Prof. Turner, en a specimen of Mesoplodon bidens, captured in 

 Shetland ; and on a specimen of Balanoptera borealis, or latireps, 

 captured in the Firth of Forth. — G. S. Shattock, note on the 

 anatomy of the Thyro-arytenoid muscle in the human larynx. 



Johns Hopkins University. Studies from the Biological 

 Laboratory, vol. ii. No. 2 (March, 18S2), contains ■ W. K. 

 Brooks, Medusa; found at Beaufort, N.C., during the summers 

 of 18S0 and 1SS1, and on the development of the ova in Salpa. 

 — J. P. McMurrich, on — the origin of the so-called "test cells" 

 in the Ascidian ovum. — G. M. Sternberg, bacterial organisms 

 commonly found on exposed mucous surfaces and in the ali- 

 mentary canal of healthy ; ersons ; — on a fatal form of Septi- 

 caemia in the rabbit from the subcutaneous injection of human 

 saliva; — on experiments with disinfectants. — H. N. Martin, 

 observations on the direct influence of variations of arterial 

 p e-sure upon the rate of beat of the mammalian heart. — W. H. 

 Howel and M. Warfield, the influences of changes of arterial 

 pressure upon the pulse rate in the Frog and the Terrapin. — H. 

 Garuian and B. P. Cnlton, notes on the development of 

 Arbacia pustulata.—K. Mitsukuri, on the structure and signi- 

 ficance of some aberrant forms of lamellibranchiale gills. — E. 

 B. Wilson, on the early developmental stages of some polychaetous 

 annelides. 



Tin- American Naturalist for April, 1882, contains — On 

 mound pipes, by E. A. Barber. — On the ll ,wers of Solatium 

 rostratum and Cassia chamaxrista, by J. E. Todd. — Is Limulus 

 an arachnid? by A. S. Packard; a criticism on the views of 

 Prof. Lankester. — On a pathogenic Schizophyte of the hog, by 

 II. J. Detmers. — On Mexican" caves with human remains, by 

 Ed. Palmer. — The Editor's table. — Recent literature.— General 

 notes, and scientific news. 



May, 1S82, contains — The acorn-storing habits of the Cali- 

 fornian woodpecker, by R. E. C. Steams. — Observations on 

 some American forms of Chara coronata, hy T. F. Allen. — The 

 loess of North America, by R. Ellsworth Call. — The ichthyo- 

 logical papers of G. P. Dunbar, with a sketch of his life by 

 J. L. Wormian. — Problems for zoologists, by J. G. Kingsley. — 

 Recent literature. — General Notes. — Scientific news. 



Verhandluugen der k. k. zoologisch-botanischen Gesellschaft in 

 Wien, Bd. xxi. Heft 2, 1882, contains : Josef Mik, diptero- 

 logical studies, II. (pi. xvi.), and notes on G. Strobl's dis- 

 coveries of Diptera at Seitenstetten. — Ed. Ritter, on the 

 P f elaphidse and Scydmaenidae of Syria ; analytic key to the 

 European Coleoptera, V. (pi. xix.). — C. R. Osten-Sacken, list 

 of the entomological writings of Kondani (supplementary to 

 Hagen). — J. Freyn, supplement to the flora of South Istria. — 

 II. IS. Mbschler, contributions to the butterfly fauna of Surinam, 

 IV. and end (ph. xvii. and xviii.). — A. Rogenhofer and Dr. R. 

 W. v. Dalla Torre, on the Hymenoptera of Scopoli's " Ento- 

 mologica Carniolica." — August Pelzeln, on the second package 

 of birds sent by Dr. E. Bey from Central Africa. — Dr. L. W. 

 Schaufuss, zoological results of an excursion to the Balearic 

 Islands (pi. xxi.). — Dr. L. Koch, the Arachnida and Myriopoda 

 of the Balearic; (pi. xx. and xxi.). — Schulzer v. MUggenburg, 

 mycologica! notes, VI. — L. Ganglbauer, analytic tables of 

 European Coleoptera (pi. xxii.). — A. B. Meyer, on birds from 

 some of the southern islands of the Malay Archipelago. — Johann 

 Bubela, list of the wild plants of Bisenz in Moravia. 



Archives des Sciences Physiques et A T aturelles, April 15. — The 

 grain of the glacier, by F. A. Forel. — Note on the extension of 



