434 



HISTORY OF LENSES AND MICROSCOPES [Cn. XII 



the ocular (fig. 23-24) which now bears his name. It was soon adopted 

 for the microscope and is to this day the most used of any. 



The Ramsden ocular was devised by J. Ramsden (1782) for the 

 telescope and like the Huygenian was adapted to the microscope. It 

 has been used especially for the ocular micrometer (fig. 22 A, 93). 



The Compensation oculars were invented by Abbe (1885-1886) 

 to go with the apochromatic objectives and to correct the residual 

 defects in the objectives (fig. 22 B, 174-175). 



700. Mirrors and condensers for illuminating objects. The 

 first objects looked at through the microscope, whether simple or 



compound, were opaque and 

 must be illuminated by light 

 falling upon their surface. 

 For this were used condensing 

 lenses, plane and concave mir- 

 rors. The origin of the mirror 

 is prehistoric. The first were 

 of polished metal and of dark 

 minerals. Those with a metal 

 backing have been known only 

 since about the i2th or i3th 

 century, and those with silver 

 only since about 100 years ago. 

 It is not to be forgotten that 

 still water and other smooth 

 objects in nature serve as mir- 

 rors, and have always existed. 

 In Descartes' picture of the 

 Dutch compound microscope 

 (fig. 251) there is a parabolic 

 mirror for lighting the object if 

 opaque, and a condensing lens 

 for transparent objects. Des- 

 cartes also gives a picture of a 

 simple microscope with a similar concave mirror for illuminating the 

 opaque object (fig. 252). In 1668 Hooke speaks of looking-glasses 



FIG. 251. DESCARTES' DUTCH COM- 

 POUND MICROSCOPE WITH A PARABOLIC 

 MIRROR AND A CONDENSING LENS. 



abc, def, Concave ocular (amplifier). 



5 T Stand and circle holding the mi- 

 croscope and pointing it toward the sun or 

 other light source. 



N O P Convex objective. 



C C Parabolic mirror for illuminating 

 opaque objects. 



i i Condenser for illuminating trans- 

 parent objects. 



