Dr Goring on Monochromatic Light, §c. 61 



met with it, because, I presume,if we use a Huygenian eye-piece 

 to form a secondary image, the component lenses will be nearly 

 of the worst figure, and in the worst position, for giving, dis- 

 tinctness, which is a quality to be preferred even to achroma- 

 tism. I need not observe that such a construction could never 

 be used by itself as an engyscope or compound microscope, 

 because its focus being negative, or between the glasses, it can 

 only be made to operate on an image already formed.* 



In the year 1815, being then a student in the University of 

 Edinburgh, I began my career as a reformer of microscopes, 

 from reading the article Telescope in the Encyclopaedia Britan- 

 nica, (written by Professor Robison,) and chiefly from the 

 consideration of a passage in it, which I shall here quote. 



" We have examined trigonometrically the progress of a 

 red and a violet ray through many eye-pieces of Dollond's and 

 Ramsden's best telescopes ; and we have found in all of them 

 that the colours are united on or very near the field-glass ; so 

 that we presume that a theory somewhat analogous to ours 

 has directed the ingenious inventors. We meet with many 

 made by other artists, and even some of theirs, where a consi- 

 derable degree of colour remains, sometimes in the natural or- 

 der, and often in the contrary order. This must happen in 

 the hands of mere imitators, ignorant of principle. We pre- 

 sume that we have now made this principle sufficiently plain. 

 Fig. 20, No. 2, represents the eye-piece of a very fine spy-glass 

 by Mr Ramsden ; the focal length of its object-glass is 8] in- 

 ches, with lj'jjth of aperture, 2° 05' of visible field, and 15,4 

 magnifying power. The distances and focal lengths are of 

 their proper dimensions, but the apertures are a-half larger, 

 that the progress of a lateral pencil might be more distinctly 

 drawn. The dimensions are as follow : — 

 Foe. lengths Aa = 0,775 B6 =1,025 Cc= 1,01 Dd = 0,79. 

 Distances AB = 1,18 BC = 1,88 CD = 1,105. 



" It is perfectly achromatic, and the colours are united, not 



. * Such an eye- piece is very foggy when acting with a large opening, though 



it must be confessed that the focuses and the position of lenses become 



of little consequence in a practical point of view, if their acting aperture is 



■ '"h('/ imall, for the i ye cannot, under Buch circumstances, appreciate 



tluir eflect. 



