ABBE, AXD MICROSCOPE THEORY IN GERMANY. 15T 



arriving at a surface depends on the angle of aperture of the 

 illuminating pencils, and the total intensity of illumination 

 depends besides on the angle which the axes of the illuminating 

 pencils make with the normals to the surface. The results of 

 the further investigations dependent on these simple laws form 

 the basis for the determination of the light intensity in all 

 optical instruments ; these include, amongst other things, the 

 general theory of illuminating apparatus, the effect of the 

 diaphragms, and the signification of the aperture images of 

 pupils,* although the latter term is not yet used in this 

 treatise. 



In the closest connection with these theoretical investigations 

 stands the construction of the illuminating apparatus which is 

 named after Abbe, and which to-day is considered as an almost 

 indispensable accessory to all good microscopes, although it was 

 really only designed by its originator for the testing of objectives 

 and for experiments on the effects of diffraction. Several practical 

 microscopists first pointed out the utility of the apparatus for 

 diverse requirements in microscopic observation. 



The formerly very prevalent view that by means of special 

 apparatus the light serving for illumination could to a certain 

 extent be condensed — hence also the name condenser — is without 

 justification, and Abbe expresses it curtly (p. 102) that no illumi- 

 nating apparatus, however ingenious, can ever give a more intense 

 illumination than the source of light itself ; the effect of such 

 apparatus is, in fact, the very reverse of that which their name 

 implies — they do not bring about a condensation, but an attenua- 

 tion, in consequence of the unavoidable reflections and refractions,, 

 which cause a loss of the available illuminating power. 



A complicated illuminating apparatus can only offer advantages, 

 because by its means the light can be regulated much more simply 

 and with greater certainty, and because it allows of much greater 

 variation in the direction of the incidence of light than can be 

 attained by illumination from a simple mirror. The light rays 

 can be made to impinge on the object from all directions if, in 

 place of issuing from a limited surface, such as the mirror, it can 



* This has reference to the so-called " entrance and exit pupils," a 

 conception introduced by Abbe for the images of the diaphragms limiting 

 the aperture, which has come to be generally used in German works treating; 

 of optical instruments. — Translator. 



