Dr AVollaston's Microscopical Doublet 323 



Art. XVII. — A description of a Microscopic Doublet. By 

 William Hyde Wollaston, M. D. F. R. S. &c. * 



In the illumination of microscopic objects, whatever light is 

 collected and brought to the eye, beyond that which is fully 

 commanded by the object-glasses, tends rather to impede than 

 to assist distinct vision. 



My endeavour has been, to collect as much of the admitted 

 light as can be done by simple means, to a focus in the same 

 plane as the object to be examined. For this purpose I have 

 used with success a plane mirror to direct the light, and a 

 plano-convex lens to collect it ; the plane side of the lens ber 

 ing towards the object to be illuminated. 



With respect to the apparatus for magnifying, notwithstand^ 

 ing the great improvements lately made in the construction of 

 microscopes, by the introduction of achromatic object-glasses, 

 and the manifest superiority they possess over any single mi- 

 croscope, in the greater extent of field they present to view at 

 once, whereby they are admirably adapted to make an enter- 

 taining exhibition of known objects, hardly any one of the 

 compound microscopes which I have yet seen, is capable of 

 exhibiting minute bodies with that extreme distinctnesss which 

 is to be attained by more simple means, and which is abso- 

 lutely necessary for an original examination of unknown ob- 

 jects. 



My experience has led me to prefer a lens of a plano-con- 

 vex form, even when made of glass ; but the sapphire lens of 

 this form, recently introduced into use by Mr Pritchard, has 

 a decided superiority over every single lens hitherto employed. 



The cost, however, of such a lens in comparison with glass, 

 •as well as the readiness with which any number and variety 

 of the latter kind can be procured, led me to consider what 

 simple combinations of them might perhaps equal the sappliire 

 lens in performance, without great cost, or difficulty of con- 

 struction ; and though both Mr Herschel and Professor Airy 

 have recently applied their superior talents to the analytical 

 investigation of this subject, it seemed not impossible that the 



• From Phi/. Trans. 1859, p. 9. 



