lO 



now TO PHOTOGRAPH 



tiire of the light used ; (4th) tlie nature of 

 the object, yellow and brown objects always 

 requiring a prolonged exposure; {5th) the 

 de\elopment. An operator who uses a 

 weak developer will always expose his 

 plates for an unnecessarily long time. The 

 beginner will perhaps think the exposure 

 the most difficult part of photo-microgra- 

 phy, but as he progresses he will alter his 

 mind, and think the proper illumination of 

 the object far more difficult. 



Using a good paraffine lamp, and lenses 

 of from 5 inches to -|\-inch, the exposures 

 may vary from a fraction of a second to 



half an hour. For instance, the larva of a 

 flea, a very transparent object when proper- 

 ly illuminated, will not require more than 

 half a second with the 2- inch objective, 

 while with the same lens a section of coal 

 may require twenty minutes. The section 

 of Alveolina limestone figured below, al- 

 though of a white color, and apparently very 

 transparent, had an exposure of fifteen min- 

 utes. As a rule, all rock sections will re 

 quire a rather long exposure, as they stop a 

 large amount of light. All sections should 

 be as thin as possible ; yet a good color is 

 of more importance than thinness. Th^ 



r.lMKSTONI", Ml UAri.T, liHl.GRj' 



writer has a section of a tertiary limestone 

 from Bengal, of a strong yellow color, which 

 he has often vainly tried to photograph sat- 

 isfactorily, although the section is most ad- 

 mirably cut, and very thin. 



The wing of a miJge, here shown, will 

 serve as a good example of a very transpar- 

 ent object, which yet has plenty of detail. 

 This oliject had an exposure of one second ; 

 less would have sufficed with a more power- 

 ful lamp. 



High powers, being used chiefly with 

 very transparent objects, do not require the 

 irtinen 'ous exposures that people generally 



imagine. A -.^^-inch. when properly illu- 

 minated, will give a good clear image on 

 the focussing screen five feet away from the 

 object. When the writer first began using 

 high powers, he heard such exaggerated 

 statements about the difficulty attending 

 their use, and the long exposures they re 

 quired, that on first trying P. angula- 

 tum, he gave an exposure of an hour to 

 this transparent object. On developing, 

 the plate came out almost clear glass; only 

 a faint ghost of the object could be seen on 

 the plate. A subsequent exposure of fif- 

 teen minutes, under the same conditions, 



