CH. XII] HISTORY OF LENSES AND MICROSCOPES 



435 



for illuminating transparent objects for projection. The first pictures 

 of compound microscopes with the mirror, as at present under the 

 stage, are by Hertzel (1712) and Marshall (1718). 



A condenser of a single lens or of a combination of lenses for trans- 

 parent objects dates from the earliest use of the compound microscope, 

 as shown by Descartes' figure. Its importance 

 for adequate lighting has never been lost sight 

 of, as indicated by Brewster ( looa) and by 

 Nelson (see in collateral reading); and never 

 so thoroughly appreciated as at the present 

 day. The form most common on microscopes 

 is the uncorrected one of Abbe which was first 

 described in the Archiv fur Mikr. Anat. Vol. 

 9, 1873, p. 469. 



701. Achromatization. As pointed out 

 in 463-464, white light, being composed of 

 different wave lengths (fig. 144-146), must be 

 differently refracted when passed through a 

 prism or lens. To the normal human eye the 

 different waves when separated or dispersed 

 out into groups appear of different colors. 

 Although the nomenclature used by Newton 

 was somewhat different from that now used, 

 he supposed that the refraction of the differ- 

 ent waves was in exact accordance with their 

 wave lengths, as is the case with a diffraction 

 grating, and hence there could be no achro- 

 matization of dioptric instruments, for when 

 the dispersion was overcome the refraction 

 must also be eliminated. The mistaken belief 

 that the human eye was achromatic, however, 

 kept alive the hope of producing achromatic 

 microscopes and telescopes. Experiments on a large number of 

 transparent substances showed that while all dispersed the light, the 

 dispersion was not the same in all, some affecting one group out of 

 proportion to another. This irregularity gave the clue to the way to 



FIG. 252. DESCARTES' 

 SIMPLE MICROSCOPE. 



1 1 Rays of light pass- 

 ing to the reflector. 



C The parabolic re- 

 flector for illuminating 

 the opaque object. 



A The plano-convex 

 lens serving as a mag- 

 nifier. 



GE Pin for holding 

 the opaque object. 



H Crystalline lens of 

 the eye. 



