146 THE HISTOKY AND DEVELOPMENT OF THE MICROSCOPE 



given in fig. 113. It was for examining transparent objects and was 

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

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

 The object was placed at M 1ST ; 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 

 used as a microscope. 



Even 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 

 by Cuff that can only be regarded as the precursor of the most com- 



FIG. 118. Smith's reflecting microscope (173S). 



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

 in fig. 114. A disc of plane glass, C, or a concave, M, was applied, 

 on the stage of which dissections &c. could be made ; a mirror, 1 , 

 was fitted in a gimbal 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 

 the pillar for focussing etc. This motion of the lens over the object 

 became very popular and was employed in nearly all microscopes up 

 to the time of the establishment of achromatism ; the last microscope 

 so fitted was that designed by Mr. W. Valentine and made by 

 Andrew Ross 1831. The movement in arc lasted much longer, and 

 the last remnant of it is still to be found in Powell's No. 1. 

 The pillar screwed on the lid of the box, within which the instru- 

 ment was packed with sundry accessories. 



It was to the discovery of achromatism as applied to microscopic 



