60 Dr Goring on Monochromatic Light, <$;c. 



tube. The aperture of the hole in it is usually about one- 

 tenth of an inch in diameter, or, just what will allow the pencil 

 of light from the object-glass to clear it. The telescope will 

 be just as achromatic without it as with it, when the eye-piece 

 is removed from the telescope and tried by itself. 



As a compound microscope, the said diaphragm acts a very 

 important part, and in fact reduces the angle of aperture of 

 the eye- piece nearly to that of the object-glass of the telescope 

 along with which it acted, being, as I said before, perhaps six 

 degrees. Now it is no wonder that with so small an opening, 

 (though unassisted by the over-corrected object-glass,) the eye- 

 piece fixed should still pass for achromatic with those who know 

 not how to look into its defects, — just as we see mulattoes and 

 people of colour who have not much black blood in their veins 

 pass for whites. Let, however, this said stop be knocked out, 

 or, what will amount to the same thing, let it be opened out 

 with a broach, until it will admit the whole of the pencil of a 

 verfectly achromatic object-glass having a large angular aper- 

 ture ; for example, one belonging to an opera-glass, or a per- 

 spective, or some of those dumpy achromatics which Ramsden 

 was in the habit of making, this will probably cause an en- 

 largement of the perforation of the said stop from T J g to f of an 

 inch, if the focus of the first glass is two inches. The ra- 

 tio between the diameter of the visual pencil, and that of the 

 object-glass will truly represent the magnifying power of the te- 

 lescope, and it may be known to have attained its due aperture 

 when none of that of the object-glass is cut by it. On trying 

 any telescope thus mounted, it will soon be seen whether the 

 eye-piece is achromatic or not. Or it may be tried as a com- 

 pound microscope by itself, and I can only say that a man who 

 will not be able to perceive colour in it, must be either utterly 

 blinded by preconceived opinions, or by a complete incapacity 

 to discuss the merits of optical instruments. Nevertheless, 

 there is a construction for an erecting eye-piece which is bond 

 fide achromatic to any angle of aperture as it appears to me, 

 namely, a simple reduplication of the Huygenian eye-piece, 

 that is using one to erect and form the secondary image, and 

 another to view it, which will give a compensation to each 

 part ; but this construction is never employed, — at least I never 



