54 Dr Goring on Monochromatic Light, $0. 



about a foot distant from it, I hung up a piece of black silk 

 velvet. I likewise drew a shade over the object-end of the 

 microscope, reaching close down to the slider, so that no light 

 should get into the body of the instrument by reflexion. By 

 these means, when its power was equal to that of a ^ of an 

 inch lens, I could not see my field of view. I then introduced 

 a prism, (the edges and other parts of which not employed to 

 refract light were carefully blacked, that no white light might 

 be reflected or radiated by it,) between the piece of black vel- 

 vet and the tube, so as to refract the light of the sun from a 

 mirror into the microscope. The prism was placed vertically, 

 so that either of the seven colours it produced could be brought 

 to bear upon the small aperture at the end of the tube, to the 

 exclusion of the rest ; the spectrum being of considerable 

 breadth, owing to the distance of the prism from the diaphragm, 

 which was about three feet. Now, by this apparatus, I could 

 very easily select any one of the colours I wanted, by merely 

 moving the microscope a little to the right or left, and illumi- 

 nate the object with it. The object-glass which I made use 

 of was a plano-convex of half an inch focus, and a quarter of 

 an inch of aperture, which, with the white light of the sun, 

 was just able to show the lines upon the scales of the Menelaus 

 butterfly in a very imperfect manner ; and was much disap- 

 pointed and surprised to find that no amelioration in the vision 

 was produced by using any of the prismatic colours instead of 

 the white light. Moreover, I found that not one of the colours 

 produced apparent achromatism, unless it truly perforated the 

 axis of the microscope; any obliquity of the light instantly caus- 

 ing prismatic colours to appear, modified by the tint of the co- 

 loured ray used, much in the same manner as white light would 

 have been if merely passed through a piece of coloured glass. 

 Now the lined objects always require oblique light for their 

 manifestation, and consequently, never could be shown without 

 exhibiting other colours beside those of the primary ray em- 

 ployed ; but other objects which agree with direct light could 

 be shown destitute of prismatic fringes, though very confused 

 and indistinct. I may observe, that the light of the sun treat- 

 ed in this way is by no means too intense for microscopic pur- 

 poses. I even thought, that the violet ray was not intense 



