146 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE 



in fig. 1 1 3. It was for examining transparent objects and v,-;is 

 -imilar to the Cassegrainian telescope, but with an extra long eye- 

 piece tube to permit the focussing by movement of the eye-lens. 

 Tin- object was placed at M X ; the image was taken up by the 

 concave, reflected on the convex, and again reflected to the eye-lens. 

 He advised the use of a condensing lens for the illumination, to pre- 

 vent ' the mixture of foreign rays with those of the object,' otherwise 

 the instrument gave confused images of distant objects when it was 

 u>cd as a microscope. 



Kven without a condenser there are good images attainable with 

 this instrument, but with the condenser they would be, of course, 

 improved. 



We have not followed in any detail the forms of simple micro- 

 scopes as they presented themselves, but in 1755 a form was made 

 1 iy C'uff that can only be regarded as the precursor of the most com- 



A- 



FIG. ll:j. Smith's reflecting microscope (1738). 



plete and perfecl of our simple dissecting microscopes : it is shown 



A disc of plane glass, C, or a concave, M, was applied, 



on the stage of which dissections .vr. could be made; a mirror, I, 



was fitted in a -imb.-d with a stem sliding in a socket in the pillar; 



the lens-carrier, F, alone, or with Lieberkiihn, F, screwed in a ring 



on the end of a horizontal arm. E. sliding through a socket, attached 



to a vertical rod, D, sliding and rotating in a socket at the back of 



pillar for focussing A-c. This motion of the lens over the object 



:came very popular and was employed in nearly all microscopes up 



mif <>f the establishment of achromatism ;' the last microscope 



was that designed b\ Mr. \V. Valentine and made by 



' l ' 1 " 1 movement in arc lasted much longer, anil 



remnant of it is still to be found in Powell's No. I. 



'" 1 "" ''"' 'id of the box. within which the instru- 



ked with sundry accessories. 



'he discovery of achromatism as applied to microscopic 



