74 PHOTO-MICROGRAPHS. 
flected light from the sky, — and using Eastman’s 
instantaneous dry plates, the time of exposure 
with a one-inch objective (Collins’s) is from one 
to two minutes when the amplification is fifty 
diameters. At the same place and under the same 
conditions, a photograph of Bacteria (B. rubescens, 
Ray Lankester), in which the amplification was 
six hundred diameters, and the objective used was 
Zeiss’s one-eighteenth-inch homogeneous immer- 
sion, the time required was twenty-five minutes. 
These long exposures would be impossible with 
wet plates; but a dry plate may be exposed for 
any length of time required, and, as already stated 
in the section on Light, any image that makes 
a sufficiently distinct impression on the retina to 
enable the operator to focus it properly, may be 
photographed. 
Professor C. H. Kain has been kind enough to 
send me some of his photographs made by lamp- 
light.’ The notes upon these show that the time 
of exposure, when the amplification was about 
twenty-five diameters, was from three to six 
minutes. 
In an under-exposed photograph of Arachnoidiscus 
ornatus, made with Wales’s one-tenth-inch objective 
and A. eye-piece, — amplification about three hun- 
dred and fifty diameters, — with illumination by 
student’s-lamp, bull’s-eye condenser, and achro- 
matic condenser (Carbutt’s J. C. B. plate), the 
time of exposure was ten minutes. 
1 See his paper in the Amer. Month. Micr. Journal, Vol. II. 
No. 4, p. 71. 
