E. M. NELSON ON EVOLUTION OF THE MICROSCOPE. 109 



the late J. May all, Juiir., in the Essai de Physique, by P. Van 

 Musschenbroek, Vol. 2, PI. 18, Fig. 6, Leyclen (1739), is inserted. 



It would seem that the conditions laid down in Descartes' 

 figure — viz. that the foci of both the mirror and objective should 

 be identical — were not adhered to, for we find the same speculum 

 used below lenses of very different foci. 



Cuff" subsequently improved this by making the mirror slide on 

 a long cylindrical nose-piece, its proper position for use with any 

 particular objective being indicated by a numbered mark cor- 

 responding to a number on the objective (see E, Fig. 23). Dr. 

 Lieberkuhn's latest development of this apparatus consisted of 

 a short cylindrical tube, at one end of which was a concave 

 speculum | inch in focus, having the usual hole at its vertex, in 

 which was mounted the lens, also | inch in focus ; at the other 

 end of the tube there was a biconvex condensing lens, the object 

 being placed on a sliding pin between the condensing lens and 

 the speculum. 



Here the speculum and lens have the same focal lengths, but 

 still Descartes' idea is not carried out, because the speculum is 

 mounted nearer the object than the lens, for the purpose of 

 focussing the convergent rays that fall upon it from the con- 

 densing lens. This, optically speaking, is a very indifferent 

 arrangement, because the diameter of the condensing lens being 

 equal to that of the speculum, the condensed rays can only fall 

 on the central portion of the speculum, which of course has not 

 much effective area owing to the hole in it. 



Dr. Lieberkuhn may perhaps have been the first to practically 

 use Descartes' invention, but all his adaptations of it were of the 

 crudest kind, and have quite passed away ; whereas Cuff''s form 

 of mounting it remains. 



To Dr. Lieberkuhn we are indebted for a solar microscope, 

 so called because direct sunlight was used as an illuminant ; now, 

 however, we should call it a projection microscope. (Exhibited 

 together with the Lieberkuhn by the Doctor himself in London in 

 1740.) Originally the apparatus, which was mounted through a 

 ball and socket in a window, was pointed directly at the sun ; but 

 this was altered and greatly improved by Cuff (1743), who 

 attached an elementary form of heliostat (see E, Fig. 19), which 

 permitted the microscope to be kept in a horizontal position, the 

 thumbscrew T causing the mirror E to rotate round the optic 



