268 Dr. Go ring's Commentary on 



proceed not far in the scale of power : the deepest which has 

 yet been made, is one by Mr. W.Tulley of-j'^^ of an inch focus, 

 and another by Messrs. Chevalier, composed of two double 

 ones, each of two French lines focus. It is true, that these 

 being intended for the object-glasses of engyscopes, are not 

 right, in point of figure, as mere magnifiers, in which way they 

 shew a want of aberration in the convex lenses, when tried 

 upon an artificial star put out of focus — a certain reduction of 

 the aperture, however, makes them right. In default of these 

 as well as for higher powers, come the doublets of Mr. Her- 

 schel, which, I think, might be executed as far as the one- 

 twentieth of an inch focus, t am afraid it would be very dif- 

 ficult to work a concave surface in sapphire^ but a doublet of 

 that substance, of proper curves, would be an iinprovement 

 on those made of glass, on account of the low dispersive power 

 of the sapphire : then come simple plano-convexes (or, if pos- 

 sible, menisci) of sapphire and diamond, of which latter sub- 

 stance a plano-convex of the one-eightieth of an inch might 

 be executed. 



As no plano-convex lens of the one-eightieth of an inch 

 focus could be formed from any other substance, such a lens 

 would, in my idea, constitute the ultimatum of the perfection 

 of the single microscope. A piano-lens of glass could not be 

 well made deeper than one-fortieth of an inch, and its aberra- 

 tion would greatly surpass that of diamond ; if powers higher 

 than one-eightieth of an inch are wanted ; the forms even of 

 diamond lenses must necessarily be equi-convex, from the ^m- 

 possihility of executing others. 



I now proceed to the consideration of the illuminative part 

 of Dr. WoUaston's instrument. 



From the coinmencement of the period in which I first ob- 

 served the property of the lined objects as tests ^, that is to 

 say, of being only visible by microscopes^ having large angles 

 of aperture, free to a certain extent from aberration, I re- 

 marked also, that the more difficult ones are only visible 



image, it will be found out one day, that these lenses, properly fi^ired, will make 

 the best sort of positive eye-piece for the telescopes of astronomical instruments, 

 in which it is no less important to correct the chromatic than the spherical aberttx- 

 tion, for no other kind of positive combination can be rendered achromatic. 



* Vide vol. xxi. p. 34, and xxii. p. 265, of the old series of this Journal ; us mere 

 microscopic objects, they were first noticed by Leeuenhoek. 



