THE LIGHT. 29 
used —at a proper distance at the back of the 
achromatic condenser, or to use a heliostat. 
In general terms, it may be said that any image 
upon the screen which makes a distinct impression 
on the retina in a dark room can be photographed. 
If the light is very feeble, and the image barely 
perceptible, it will be necessary to use a very sen- 
sitive plate, and to make a long exposure. 
The writer has made a good photograph of Bac- 
teria (B. rubescens, Ray Lankester) with an amplifi- 
cation of six hundred diameters, without a heliostat, 
using a dry one-sixth-inch objective of Zeiss as a 
condenser, and obtaining the light directly from 
the blue sky. The objective used was Zeiss’s 
one-eighteenth-inch homogeneous oil-immersion, 
and the time of exposure was half an hour, East- 
man’s instantaneous dry plate. 
From five to six hundred diameters is proba- 
bly about the limit with light reflected from the 
sky, and with such condensing apparatus as is 
commonly supplied by dealers in optical apparatus 
for ordinary use with the microscope. The writer 
can see no good reason why the opticians should 
not be able to construct an achromatic condenser 
for photography which by collecting the light from 
a larger area would give sufficient illumination for 
the highest powers, and thus enable the operator 
to dispense entirely with the use of a heliostat, 
which with its accessories is a somewhat trouble- 
some and expensive piece of apparatus. 
It must be remembered that the actinic power 
of the light from the sun varies greatly at differ- 
