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11. A desctnption of a microscopic doublet. By William Hyde Wollaston, 



M.D. F.R.S. 8sc. 



Read November 27, 1828. 



J. HE State of my health induces me to commit to writing, rather more hastily 

 than I have been accustomed to do, some observations on microscopes ; and I 

 trust, that in laying them before the Royal Society, they will meet with that 

 indulgence which has been extended to all my former communications. 



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 ex- 

 amined. 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 being 

 towards the object to be illuminated. 



With respect to the apparatus for magnifying, notwithstanding the great 

 improvements lately made in the construction of microscopes, by the introduc- 

 tion of achromatic object-glasses, and the manifest superiority they possess over 

 any single microscope, in the greater extent of field they present to view at 

 once, whereby they are admirably adapted to make an entertaining 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 distinctness 

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

 sary for an original examination of unkno^vn objects. 



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



MDCCCXXIX. c 



