PRACTICAL PHOTO-MICROGRAPHY. 173 



The successive operations towards " setting up " this object 

 may be thus described. A suitable frustule is chosen, a me- 

 dium power objective being used to centre the object and the 

 light, and to focus the condenser accurately upon the object. 

 The slotted stop is then inserted in the condenser, and the 

 effect examined with the medium power ; little else beside the 

 frustule should be illuminated. Both high-power objective 

 and condenser must be oil immersions and of the best quality. 



The high-power objective is now put into place and action, 

 and the most accurate correction by collar or tube length ob- 

 tained by experiment. The condenser will almost certainly 

 require re-focusing after the high-power glass is brought into 

 play. With perfection of apparatus the white lines ought to 

 appear about four times as broad as the black ones (Nelson). 



The focusing of the image on the screen is with ordinary 

 achromatic lenses exceedingly difficult, with apochromatics 

 less so. No pains should be spared to get a correct focus, 

 and after it is got the whole apparatus should be allowed 

 to rest for ten minutes at least, the light at full blaze ; after 

 this time the image is to be again examined to test for steadi- 

 ness. Clearly if the accuracy is lost in ten minutes it is hope- 

 less to attempt an exposure of twenty minutes to an hour with 

 lime, not to speak of four to ten hours with an oil lamp. "We 

 have exposed for six hours on this diatom with oil, and our 

 usual time with lime is twenty-five to thirty minutes ; with 

 these exposures we expect to get a good photographic negative ; 

 if there has been no motion of the image we are much pleased 

 and somewhat surprised. There is no special difficulty in de- 

 velopment. 



In spite of all these details, A.pellucida is child's play to 

 photograph in comparison with such tests as P. angulatum, S. 

 gemma, and N. rhomboides by axial light and to show " black 

 dots." P. angulabum in white areoles, or N. rhomboides in 

 squares with a special disc in the condenser, is infinitely easier 

 than the same in black dots. 



