136 
PRACTICAL PHOTO-MICROGRAPHY. 
distant from an ordinary gas burner, or for a shorter 
time to an incandescent (Welsbach) burner ; but most 
even, certain, and speedy is the use of magnesium 
ribbon. A very good plan is to ascertain the density of 
each negative by means of a small and cheap instru¬ 
ment on the market, known as a “ Dawson’s Densi¬ 
tometer ” (Messrs. Geo. Houghton and Co.), and by 
experiment to discover what length of ribbon at what 
distance is required for a negative of that density. The 
instrument costs only five shillings, and the observation 
is made in a few seconds, the densest part of the 
negative being chosen for the observation. For an 
example we find that a negative of “ density 2 ” 
requires one inch of ribbon at fifteen inches distance, 
and all negatives registering that density require the 
same. It need hardly be pointed out that yellowness 
in a negative will entail more exposure. 
A great many firms are now selling papers of this 
class, but we take as our example the “ Velox ” of the 
Nepera Co., of New York State, for which Messrs. 
Griffin, of Sardinia Street, London, are the agents, as 
this paper is perhaps the best known. Kodak Limited, 
however, sell a similar paper called “Dekko”; the Paget 
Prize Plate Co. have their “ Gravura ” ; and there are 
others too numerous to mention, all of them very similar 
so far as we have observed. 
The exposure having been made to strong light as 
above,, development may be effected in gas or lamp 
light subdued, by a solution such as the following: 
