Dr. J. Rcade on a next) Theory of Telescopes. 27 



each surface: hence they are sent magnified to (he eye-glass 

 DE, which again transmits the image to the eye Fj thence 



to the retina, and finally to the sensorium. This is a correct 

 outline of the Galilean telescope, supported in every stage by 

 direct experiment. I have particularized Dr. Young's opi- 

 nions, both because that gentleman is justly celebrated in the 

 scientific world, and because I preferred the living to the 

 dead. The pleasure I feel at having discovered the corneal 

 theory of vision, is somewhat diminished by considering that 

 it strikes at some of the most interesting theories in astro- 

 nomy. For if the theory of refraction be proved fallacious, 

 it follows as a consequence, that the greatest astronomers have 

 no idea of the distances of the planets, &c, all their calcu- 

 lations being built on visual angles and virtual images. 



On looking over the Philosophical Transactions tor 1821, I 

 find a paper written by a very intelligent experimenter, Mr. J. F. 

 Herschel, On the Aberrations of Compound Lenses and Ob- 

 ject-glasses. This gentleman,— seemingly unacquainted with 

 the experiments I have published some years ago, both in your 

 Journal and in the Experimental Outlines, — alter remarking 

 on Euler and Dalembert, from whose exertions he says m> 

 thing resulted beyond a mass of complicated formula}, he 

 gives a number of algebraic calculations, entirely built on the 

 gratuitous assumption that Newton really separated the whole 

 light into seven coloured rays. Was Mr. Herschel unac- 

 quainted with my experiments ? If so, I would beg leave to 

 call his attention to the following. 



If a square piece of black cloth with a small semicircular 



piece cut from the lower part in the following manner, r 1 



be pasted on a pane of glass at the window, on look- [ ^ J 

 ing at it through the lower refracting angle of the prism, we 

 perceive the bottom to be fringed with red and yellow rays 3 

 if we now paste another similar piece some little distance 

 below it on the same pane, and in an inverted position, as 

 thus represented 1 — \^j~~ 1 we perceive, on looking through the 



prism, the upper L, I part to be fringed with blue and 



violet: thus we have three simple and one compound colour 



produced by these fringes, derived from the black light reflected 



D 2 from 



