and on the Memoir of Enlcr. 243 



found no single object-glass could embrace a pencil of light of 

 sufficient size to render them visible. I appeal to opticians 

 whether telescopes have not been greatly improved since it has 

 become the fashion to try them on double stars. Formerly they 

 were considered good if they showed the moon and the planets 

 well ; but a much greater degree of perfection is now required 

 of them, because they have to deal with what are to them 

 proof-objects. 



Had we remained to this day ignorant of proof-objects, op- 

 ticians might certainly have made achromatic object-glasses for 

 microscopes ; but they would have been the poorest things ima- 

 ginable, for the value of a large aperture would never have 

 been felt, and when that of achromatics is reduced to a level 

 with that commonly employed in the old compounds, their 

 value or superior utility can hardly be appreciated, because 

 the aberration of the common instruments is in this state in- 

 visible to ordinary eyes. 



When I first took the subject in hand, I mentioned it to 

 perhaps the first theoretical optician of which this country can 

 boast. He was, moreover, in the habit of observing with the 

 microscope, and the remark he made me was this: " Why, I 

 always thought compound microscopes were achromatic, 1 '' To 

 which I answered, " that they might appear achromatic un- 

 der certain circumstances, provided the object-glasses had an 

 exceedingly small aperture, as was generally the case." Now, 

 if such a man could form such an opinion, what may be ex- 

 pected from the vulgar ? 



L. This is the date I think of the first effective double-object 

 glass. Though the error of the oblique pencil is so great in a 

 glass of this description, if it has a sufficient aperture to show 

 proof-objects, that all objects out of the pale of a few degrees 

 from the centre of the field of view would be completely con- 

 fused, it is this defect which renders a double object-glass un- 

 fit to be used, except in combination, unless its aperture is very 

 much reduced. 



M. In the autumn of 18213, Mr W. Tulley commenced his 

 labours on my account, and about the 1st of March 1824, pro- 

 duced an object-glass of 0.333 focus and 0.2 of aperture ; for 

 from the very beginning Tulley always aimed at the utmost pos- 



