SOME SPECIAL PBOCESSES 251 



It is therefore possible to allow the light to act on such organisms 

 or tissues, and to observe what happens with an ordinary 

 microscopic objective corrected only for white light. In this 

 case the ultra-violet light plays no part in the formation of 

 the image, but is only instrumental in producing the effect 

 which is to be observed. 



The objectives made for direct use with ultra-violet light 

 are called ' monochromats,' and the wave-length for which 

 these are corrected is 275 w. The N.A. of the highest- 

 power lens of this series is 1-25 ; but by virtue of the shortness 

 of the wave-length of the light, the resolving-power is equal 

 to an objective used with white light of 2'5 N.A. an objective 

 which of course at the present time it has not been possible to 

 produce. 



The lenses of these monochromatic objectives are made 

 exclusively of fused quartz, so that they are transparent to the 

 whole range of the ultra-violet spectrum ; that is, they transmit 

 all radiations until the point is reached where light is of such 

 short wave-length that it is absorbed by the atmosphere. 



The two highest-power objectives are immersion-lenses, the 

 immersion- fluid being a mixture of pure glycerin and distilled 

 water used in such proportions as to be of the required refrac- 

 tive index. The cover-glasses and the slides also consist of 

 fused quartz ; or, alternatively, the slides must be made of uviol 

 glass, which is sufficiently transparent to ultra-violet for most 

 purposes. 



A series of oculars, the lenses of which are also made of 

 quartz, serve to project the image on to the photographic 

 plate, and the numbers by which these oculars are known 

 indicate their magnifying power, exactly in the same manner 

 as in the case of compensating oculars. 



It is convenient to use a photographic apparatus of the 

 vertical type ; in fact, the Zeiss vertical camera is the one 

 usually recommended for this purpose, and is made in a suitable 

 form with certain modifications from the ordinary type. 



For direct observation that is, for finding the object, centre- 

 ing the image, and for preliminary observation a searcher 

 eye-piece is employed. It is so arranged above the microscope 

 that it can be used for observation and easily swung out of 

 position when the use of the camera is required ; either appliance 



