230 



PHOTOGRAPHING WITH THE MICROSCOPE [Ch. VII 



projection ocular the' tube-length is practically extended to the 



focusing screen, and as the efifect of lengthening the tube is the same 



as thickening the cover-glass, the adjusting collar must be turned to 

 a higher number than the actual thickness of the 

 cover calls for (see § 134)- 



§ 354. . Photographing without an ocular. — 

 Proceed exactly as described for the lower power, 

 but if the objective is adjustable make the proper 

 adjustment for the increased tube-length (§ 134). 

 § 355. Photographing with a projection ocular. 

 — Proceed as described in § 349 h, only in this 

 case the objective is not to be adjusted for the 

 extra length of bellows. If it is corrected for the 

 ordinary ocular, the projection ocular then pro- 

 jects this correct image upon the focusing-screen. 

 § 366. Photo-micrographs at a magnification 

 of 500 to 2000 diameters. — For this the homo- 

 geneous immersion objective is employed, and as 

 it requires a long bellows to get the higher mag- 

 nification with the objective alone, it is best to 

 use the projection oculars. 



For this work the directions given in § 104-107 

 must be followed with great exactness. The edge 

 of the petroleum lamp flame is sufficient to fill the 

 field in most cases. With many objects the time 

 required with good lamplight is not excessive; 

 viz., 40 seconds to 3 minutes. The reason of this 

 is that while the illumination diminishes directly 

 as the square of the magnification, it increases 

 with the increase in the numerical aperture, so 



that the illuminating power of the homogeneous immersion' is great 



in spite of the great magnification. 



For work with high powers a stronger light than the petroleum 



lamp is employed by those doing considerable photo-micrography. 



Good work may be done, however, with the petroleum lamp. 

 It may be well to recall the statement made in the begimiing, that 



Fig. 137. Pro- 

 jection Ocular in 

 Section. 



(About J natural 

 Size). 



1 The upper or 

 eye end of the oc- 

 ular; it is composed 

 of two convex and 

 one concave lens 

 and serves to pro- 

 ject the real image 

 formed by the ob- 

 jective and field lens 

 at d upon the screen 

 or photographic 

 plate. It is mov- 

 able to permit of 

 focusing at differ- 

 ent screen distances. 



2 Field lens of 

 the projection oc- 

 ular. 



d Diaphragm 

 where the real im- 

 age is formed. 



