CH. VIII. } 



PHOTO-MICROGRAPHY. 



203 



Fios. [60-161. Fine tint, half-tone reproductions of photo-micrographs of sections 

 made by Mrs. Gage, to show the possibilities of photo-micrography with photo- 

 graphic objectives and with low microscopic objectives without a projection ocular. 



1. Frontal section of the head of a large red Diemyctylus viridescens {red newt) 

 at the level of the portae of the brain, magnified 10 diameters. Negative made 

 with a Gundlach perigraphic objective of about go mm. equivalent focus. 



2. Frontal section of a larval Diemyctylus about jo millimeters in length. 

 Negative made with a Winkel objective of 22 millimeters equivalent focus ; no 

 ocular. Magnified 50 diameters. By the permission of Mrs. Susanna Phelps 

 Gage, from the Wilder Quarter Century Book. 



lows from the tube of the mieroscrope is noted, say on a record table 

 like that in section 348, one can get a very close approximation to the 

 power at some other time by using the same optical combination and 

 length of bellows. 



Fig. 159. Rack for drying negatives. {From 

 the Rochester Optical Co. ) 



§ 347. Photo-Micrographs at a Mag- 

 nification of 500 to 2000 Diameters. — 



, ... r For this the homogenous immersion objec- 

 \/J)lj}|Bi §|jjpllll ll|liSy tives slmuld be employed, and as it would 



'' y ~ ' ~ " require a long bellows to get the higher 



magnification with the objective alone, it is 

 best to use the projection oculars. 

 For this work the directions given in § 342 must be followed with 

 great exactness. The edge of the lamp flame will be sufficient to fill 

 the field in most cases. With many objects the time required with 

 good lamp light is not excessive ; viz., i x / 2 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 numerical aper- 

 ture, so that the illuminating power of the homogeneous immersion is 

 great in spite of the great magnification (§ 31). 



For work with high powers a stronger light than the petroleum lamp 

 is employed by those doing considerable photo-micrography. Very 

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



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

 the specimen to be photographed must be of especial excellence for all 

 powers. No one will doubt the truth of the statement who undertakes 

 to make photo-micrographs at a magnification of 500 to 2000 diameters. 



