NATURAL PHILOSOPHY. 201 



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retina, must be deprived, to a greater or less extent, of its actinic rays (by 

 passing through the yellow spot) before it determines a luminous sensation, 

 unless the portion of the retina under consideration differs from all other trans- 

 parent media known to us, in not arresting the chemical rays. If it be not hi 

 this respect exceptionable, then the theory of perfect human vision may be 

 simplified by the exclusion from consideration of the actinic rays ; and one 

 use of the yellow spot, for which it has hitherto baffled physiologists to find a 

 use, may be to extinguish these radiations. It may be further remarked, in 

 reference to the absorption of the actinic rays by the yellow spot, that the 

 views of those who have described visual impressions on the retina as phe- 

 nomena of the same kind as photographic impressions on chemically prepared 

 surfaces must fall to the ground, if the actinic rays of light are stopped before 

 reaching the optically sensific constituents of the retina. The similar opinion, 

 also, that " spectral vision" and other abnormal peculiarities of sight, are 

 phenomena of the same kind as the development (as it is technically called) 

 of latent photographic images, must, for the reason mentioned, be abandoned. 

 It will still of course be competent to compare normal and abnormal vision 

 with photographic effects, as phenomena displaying analogy, though not 

 affinity. 



One other relation of the retina to light may be referred to. If only these 

 rays which are reflected, from the choroid produce by their impact on the 

 retina the objective perception of light, and if the depth of tint of the yellow 

 spot be considerable and its color at once homogeneous, then perfect vision 

 must be exercised by yellow, and not by white light. But if this be the case 

 we should be unconscious of red and blue when seeing best, or at least 

 should receive from them an impression very different from that which they 

 occasion when they affect the general surface of the retina, 



NEW FOIUI OF TELESCOPE. 



At the American Association for the Promotion of Science, Mr. Alvan 

 Clarke, of Cambridge, gave a description of a new instrument of his own 

 invention, for measuring the distance apart of stars too distant to be brought 

 into the field of view of a telescope. Within a year from the first thought 

 of the instrument entering his mind, he had built a telescope of six inches 

 aperture, and 103 inches focal length, mounted it equatorially, governing its 

 motion by Bond's spring-governor clock, provided the two eye-pieces, and as 

 a substitute for a filar-micrometer, arranged a mode of using pieces of glass 

 ruled with a ruling machine. Experiments have demonstrated the feasibility 

 of using the two eye-pieces in this way, and of obtaining by them very 

 accurate measures of the distances of stars, which are from three to one 

 hundred minutes of space apart. The success of the instrument was, how- 

 ever, greatly due to the spring-governor which keeps each star upon the wire 

 accurately bisected. 



Professor Pierce said there were two other things besides the double eye- 

 piece in this instrument, to which attention should be directed. The eye- 

 piece must be tested by experiments, but the new mounting of the telescope, 

 a modification of the Munich, was exceedingly beautiful, more so than even 



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