Dr. fVollastori'a Microscopic Doublet 263 



possible system of curves for them : for it is the peculiar glory 

 and privilege of science to do this, and it is to be hoped that 

 their highnesses, the mathematicians, will one day cast a spell 

 for that purpose. It is curious to see how they have, with the 

 exception of Euler, blinked the subject hitherto. As to micro- 

 scopic doublets, there are many theories on the subject, and I 

 suspect that a man who sets up a candle in the blaze of the 

 meridian sun does as wise and as useful an action as he who 

 experimentally treats a subject which has been analytically 

 examined by Messrs. Herschel and Airy. We shall see, by 

 and by, whether Dr. Wollaston's labours have been rewarded 

 by any useful result or not. 



Par. 7. " The consideration of that form of eye-piece for astronomical 

 telescopes, called Huygenian, suggested the probability that a similar 

 combination should have a similar advantage, of correcting both chroma- 

 tic and spherical aberration, if employed in an opposite direction as a 

 microscope /" 



It is very possible that Dr. Wollaston may here have risen a 

 full octave above my dwarfish comprehension, for my education 

 has been sadly neglected in the exact sciences. The paragraph 

 quoted, contains the theory of his microscopic doublet, or what 

 he at least considers such. What is the Huygenian eye-piece 

 but a microscope for viewing an image 9 Its property of cor- 

 recting chromatic aberration depends upon its field-glass being 

 negative, and detracting from the magnifying power of the 

 other, as a concave lens might do. 



The forms of its component lenses, considered apart, are 

 such as give a handsome field of view, rather than correct 

 spherical aberration, though they certainly give its emergent 

 pencils parallel, when operating with an object-glass*. As to 

 reversing or inverting ity its optical constitution is utterly 

 subverted and destroyed by this operation ; in fact, it never can 

 be employed to magnify anything but an image, its focus being 

 between the glasses. 



* I observe that all those individuals who have given the construction of erecting 

 eye-pieces of four glasses to effect the greatest possible reduction of the spherical 

 aberration, always select forms similar to those of the Huygenian eye-piece, as the 

 best to correct, along with the other two (which perform the part of an object-glass, 

 and cause the rays to impinge on the ocillar ones, much in the same state as if 

 they had proceeded from the objective of a telescope ;) it is probable, therefore, that 

 an Huygenian eye-piece, ticting as a part of a telescope or engy scope, occasions no 

 spherical aberration. 



