6l2 



SCIENCE. 



and — lYz , my eyes being perfectly healthy. If a curve 

 be constructed from the formula, the values of a being 

 taken as abscissas and those of D as ordinates, for par- 

 allelism of visual lines we have a = o, D = eo , and the 

 axis of ordinates is hence an asymptote. A vanishing 

 point should therefore be reached by the external binoc- 

 ular image. Its apparent distance, however, is still finite, 

 and vision very easy. In passing to negative values of 

 a by increasing the stereographic interval, the distance 

 est ; mated continues to grow in a positive direction. This 

 is undoubtedly due to the sense of increasing relaxation 

 of the internal rectus muscles and contraction of the ex- 

 ternal rectus ; but the rate of growth bears no recogniz- 

 able relation to any successicn of values given by the 

 formula. 



The explanation just given is based upon experiments, 

 the description and discussion of which must be with- 

 held for the present. It may be simply stated that with 

 binocular fusion of images from the same pair of conju- 

 gate pxtures, I have tested the visual effect ot varying 

 the optic angle from — 5 to +45 , vision becoming indis- 

 tinct after the last named limit is passed. The value of 

 the optic angle has been found to be largely, but by no 

 means exclusively, effective in determining apparent dis- 

 tance in the stereoscope, especially for convergence of 

 visual axes. Its-effect is antagonized by the difficulty of 

 focal adjustment and by the constancy ot the visual angle, 

 the latter element being particularly important when the 

 axes diverge. 



The variation in apparent magnitude of the combined 

 image, dependent upon the value of the optic angle, has 

 been noticed by Wheatstone Helmholtz ( 13 ), and Mey- 

 er ( 14 ). Helmholtz construc'ed his telestereoscope ( 15 ) 

 for producing exaggeration of perspective when distant 

 objects are viewed, but no reference is made, in this con- 

 nection, to divergence of visual lines. The possibility of 

 fusion by optic divergence seems to have been first no- 

 ticed about i86o,by Burckhardt ( 16 ); and Helmholtz notices 

 the exaggeration of apparent distance thus produced, but 

 explains it by saying ( 13 ), " Infinity does not, in our vis- 

 ual conceptions, present an impassable limit. When our 

 eyes occupy a position which is never presented in the 

 normal observation of real objects, all that we can do, 

 conforming ourselves to the rule which we ordinarily 

 follow for the interpretation of abnormal sensations, is to 

 compare the sensation produced with that which resem- 

 bles it most, and which is distinguished from it only by 

 more feeble convergence, that is, with what is given us by 

 real objects very remote." Vision in the stereoscope is 

 always to some extent abnormal. The error into which 

 Brewster fell, and in which he has been generally followed, 

 was in supposing that under such conditions no modifica- 

 tions would be imposed upon the mathematical law found 

 applicable to normal vision, in which there is perfect coin- 

 cidence between the impressions traceable to the optic 

 angle, focal adjustment, and visual angle respectively. 



W. Le Conte Stevens. 



The German government is considering the participation 

 of German men of science in the plan of International Polar 

 Research. The Reichstag has been asked to grant the 

 necessary funds $75,000. 



REFERENCES. 



("). " Science," Nov. 19th, 1881, and American Journal of Science for 

 Nov., 1881. 



(•). Helmholtz, Opt. Phys., p. 878, ei passim : or, Le Conte, Sight, 



pp. 90-144. Appleton. 1881. 

 ('"). Hcring, Beitru'gc ztir l'hy-iologie, pp. 35-64, 1861 ; or, Le Conte, 



Si^ht. pp. 2x3-361, 1881. 

 ("). Helmholtz, Opt. I'hys., pp. 935-937," 873-877, 961-964. 

 C'). Phil. Mag., 1852, p. 507. 

 C 3 ). Opt. I'hys., pp. 823-828. 

 C 4 ). Pofig. Ann., lxxxv., p. 198. 

 ("). Opt. Phys., p. 821. 



("). Verhandl. d. Naturforsch. Oics. zu Hasel. I, 145; or, Opt. Phys., 

 p. 827. 



FOREIGN NOTES ON THE SOURCE OF COMETS' 

 LIGHT. 



Numerous observations have been made abroad upon 

 comet b, 1881, to settle the question as to the origin of 

 the light cf these bodies. Messrs. Thury and Mayer at 

 Geneva compared the brightness of theccmet's head, as 

 ascertained by photometric measurements, with the 

 brightness it would have had if its light had been derived 

 solely from the sun, by reflecticn. It was found that the 

 intensity of the light of the nucleus, as it withdrew from 

 the sun, diminished at first faster, and after a time slower 

 than would have been the case, had it shone solely by 

 reflected light. The decrease in intensity took place, in 

 fact, as if the nucleus, during its approach to perihelion, 

 had acquired through the force of the sun's rays an intrinsic 

 light, which accompanied violent action of some character ; 

 this violent action ceasing after the comet had measured 

 some distance on its return track, its light decreased 

 speedily in conformity therewith, but the nucleus contin- 

 ued to glow as if in a state of incandescence, and re- 

 mained visible, according to the above observers, longer 

 than could have been expected. 



This method seems to be well adapted to an independ- 

 ent determination of this interesting question. In the 

 data it is a question, not of absolute, but of relative 

 quantities. Ignorance of the physical condition and na- 

 ture of a comet's reflecting surface renders it impossible 

 to compute the intensity of its light under reflection alone, 

 with any degree of certainty. As not the absolute light, 

 however, but the increase or decrease under the circum- 

 stances, is required, the necessity for such knowledge is 

 eliminated. 



Another conclusion as to the origin of the comet's 

 light has been reached by Respighi, Irom spectroscopic 

 evidence. According to him there is no doubt that the 

 light in part is reflected, as is proved by the appearance 

 of the Fraunhofer lines in the photcgraphs of the comet's 

 spectrum. As to the bright lines or bands, they also 

 may be caused by reflected light, as will be seen when it 

 is taken into consideration what changes th s light must 

 have undergone after it has passed through the gases 

 and vapors which form the whole mass of the comet. 

 " It is certain," he continues, " that the largest part of 

 the light emitted by the comet comes from its interior, 

 and that it has passed through thick strata of gases and 

 vapors. It is there subject to the selective absorption 

 which is peculiar to these vapors and their combinations. 

 It is accordingly natural that dark lines and bands should 

 thence arise, which are different from the Fraunhofer 

 lines ; and with the weak, but complete, spectrum of the 

 light that is reflected from the exterior substance of the 

 comet, another spectrum must appear, which is consid- 

 erably modified through powerful absorption." 



" The limits of a simpie notice do not permit me to 

 enter in detail into my numerous spectroscopic observa- 

 tions of the Comet b, 1881. But I can affirm that it does 

 not require the supposition of an intiinsic light to explain 

 the phenomena which they exhibit. For the discontinu- 

 ity of the spectrum might arise from the same cause as 

 the broad, dark bands in the spectrum of the sun near 

 the horizon, or in those of the planets. In the case of 

 comets, however, the phenomenon is greatly exaggerated 

 by the immense thickness of the absorbing strata, the 

 rich character of their chemical constitution and the weak- 

 ness of the light which they reflect to us. One must 

 therefore proceed as in the case of the spectrum of our 

 atmosphere, and not consider so much the bright bands 

 as those dark through absorption." 



A committer has been formed at Reggio,(Emilia) to collect 

 funds for establishing a fitting monument to the memory of 

 the Padre Secchi, in the form of a fine refractor, of which the 

 objective is to have 70 centimetres diameter. Reggio thus 

 follows the example of Arcetri, where a fitting scientific 

 monument has been erected to the memory of Galileo. 



