10 
The metol and hydrochinone should be dissolved in the water 
before adding the sulphite. This preparation keeps indefinitely, 
and may be used until exhausted. Whatever the exposure, the 
development should be carried out until the plate is completely 
developed ; errors in over exposure or under exposure may then 
be corrected by reduction or intensification, either process with 
full development i3roducing a better negative than one poorly 
developed. 
Formulae containing a mercury salt, for intensification, or con- 
taining potassium ferricyanide, for reducing, usually accompany 
all makes of plates. 
COST OF TEMPOEARY AXD PERMAXEXT APPARATUS. 
The advantage of this apparatus is the low primary cost. A 
lantern-slide camera at a cost of 810 makes an excellent bellows 
when firmly mounted on a frame. The generator, burner, two 
condensers and light filter would cost about 825, making a total 
cost of about 835. 
A complete outfit with the necessary mechanical arrangements 
for centering the light, varying the focus of the condensers, and 
time shutter should be supplied for about 860. 
Where a laboratory is supplied with electricity, that illuminant 
may be used by substituting a hand-feed electric arc and a large 
absorbing cell at an increase in cost of 85 to $10. 
With this apparatus and the well-known apochromatic objec- 
tives and projective oculars of Zeiss, it is possible to take a photo- 
graph of 1,000 diameters in two seconds, or about as rapidly as an 
exposure can be accurately made. For the low diameter the light 
is stopped down until the desired illumination is obtained. Our 
experience is that an exposure of five seconds for all magnifications 
gives greater detail than a more rapid exposure. In this time 
vibrations do not seriously interfere with sharpness of outline. A 
printing-out silver paper gives the best tone to a photograph when 
half-tone reproduction is desired. The finished print should be a 
shade darker, as density is always lost in the process of repro- 
duction. 
O 
