ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
653 
steam. The apparatus is heated by means of a cylindrical closed pipe, 
projecting obliquely from the lower pan. 
The agar having been cut up is boiled for half an hour on the open 
fire, and then having been mixed with any desired substances is placed 
in the funnel. In the funnel is fitted an ordinary filter-paper, filled with 
siliceous sinter. When the lid is screwed down, and heat applied, the 
pressure of the steam serves to drive the liquefied agar through the 
funnel into a flask placed underneath. 
The chief advantages of this apparatus are its rapidity, — a litre of 
2 per cent, agar can be filtered in two hours, a great economy of gas, 
simultaneous sterilization and clarification. 
(4) Photomicrography. 
The Value of using different makes of Dry Plates in Photo- 
micrography.* — Dr. W. C. Borden remarks : — “ While the variation in 
rapidity of different makes of plates is pretty generally understood and 
taken advantage of in practical work, the variations of plates in contrast 
and range of tones are not generally discussed in photographic literature, 
nor are the great benefits to be obtained by taking proper advantage 
of these variations understood, or generally practised. Hardly a photo- 
graphic journal appears without either some new formula for a developer, 
or some new method of working an old one, by which it is claimed that 
some modification of rapidity or contrast may be produced in the plate 
on which they are used. Quite a large portion of photographic litera- 
ture is devoted to giving these means of producing required effects in 
negatives, and every box of plates contains information (?) how to obtain 
greater or less rapidity, or contrast, as may be desired ; when in fact, 
after a light has once struck a plate in a particular way, so changing a 
particular ratio, the molecular structure of the sensitizing chemicals 
with which it is coated, but little change in result can be produced by 
any developer, however much that developer may be modified. A 
modification, however, of the coating of the plate, giving a different 
chemical basis upon which the light acts, will, from the different 
arrangement and kind of molecules acted upon, produce a different 
result whatever developer may be employed. It is in this way that 
variations in result may be best and most surely obtained, for different 
makers of plates use sensitizing formulas differing in such manner that 
the coatings, when acted upon by light and “ developed,” give results 
differing in rapidity, contrast, and range of tones. That almost universal 
advice: “Get a good plate, master its peculiarities, and then use this 
plate exclusively,” is good only so far as getting a, good plate and 
mastering its peculiarities are concerned, for, however well the working 
of any one plate may be understood, results cannot be obtained from it 
alone, upon all kinds of objects, equal to those obtainable when different 
makes of plates are intelligently used, in a manner to make their peculi- 
arities bring out, in the resulting negative, the effect sought for. For 
instance, if the object to be photographed has but little contrast, and a 
plate giving great contrast and a short range of half tones be used, a 
good printing negative will usually be obtained, while, if a plate having 
* Amer. Mon. Micr. Journ., xii. (1891) pp. 109--72. 
