and on some peculiar Properties ofUglit. 299 



P. S. — The very ingenious experiments of Captain Kater (in the 

 last vohime of the Philosophical Transauctions) on the different 

 quantities of light reflected by the Gregorian and Cassegrain 

 telescopes with specula of the same aperture, appear to prove 

 that when the rays of light converge to a focus, a considerable 

 part of them are (as far as affects the sight) extinguished before 

 thev diverge again. I do not know whetlier similar experiments 

 have been made with refracting telescopes, with a view to confirm 

 this fact; but there is one case well known to the earliest optical 

 writers that has hitherto been unexplained, but which I conceive 

 pffers a fiill and decisive confirmation of the same phaenomenon. 

 In the original telescope of Galileo with a concave eye-glass, 

 the rays are intercepted before they come to a focus; and 

 though this construction was laid generally aside on account of 

 the small field of view ; yet such is the superior brilliancy of the 

 effect, that Jupiter's satellites may be seen with a common re- 

 fracter of twentv inches in length, with a concave eye-piece, more 

 distinctly than with one of four feet when a convex eye-glass is 

 used. Having when a school-bov amused myself with the con- 

 struction of telescopes of this kind, I was well acquainted with 

 the effect : on referring to optical writers for the cause, I wa"> 

 informed that it arose from the concave glass being much thinner; 

 and bv later writers, that the effect was produced by the flint glass 

 of the eye-piece correcting in some degree the aberration from 

 the different refrangibilitv of the rays, as in the achromatic ob- 

 ject glass. Without denying that both the above causes may con- 

 tribute something to the superior brilliancy of the object, I am 

 persuaded that the solution which is offered by Capt. Kater's 

 experiments is the only one that will fully account for the dif- 

 ferent effects of the convex and concave eye-glasses in refracting 

 telescopes. It is much to be desired that Capt. Kater, or others 

 who have leisure for such experiments, would prosecute these 

 Inquiries, as they cannot fail to contribute much to the improve- 

 ment of optical instruments. They will also elucidate one of 

 the most interesting su[)jects of philosophical investigation, re- 

 specting the effects produced by the rays impinging on different 

 surfaces, as well as on each other under different angles, and the 

 still more extraordinary effect of their impact on each other at 

 they converge to a focus. Tliose who have attended to the 

 experiments of M. Malus and Dr. Brewster will immediately 

 perceive the importance of Capt. Kater's discovery in reference to 

 the phaenomena of the polarisation of light. If the particles of 

 light have each a positive and negative or attracting and repel- 

 ling pole, as certain experiments appear to indicate, may not the 

 oppoti.ig poles o\ some of the particles be brought so near t« 



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