656 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
graphed, which will give with a Seed plate a certain contrast in the 
negative, will with a Harvard plate give practically double the contrast. 
This comparison shows at once that the Harvard is the better plate 
to use when objects having little contrast are to be photographed, or 
when contrast is desired ; and the Seed is the better plate when rapidity 
is desired, when an object having strong contrasts is to be photographed, 
or when strong contrasts are to be avoided and a “ soft ” negative desired. 
x\lso, that by the intelligent use of these plates, or others having 
similar qualities, results may be arrived at which could not be obtained 
by the exclusive use of either alone. 
I have called attention to these particular plates, and have used 
them in illustration, because they have the opposite qualities, by taking 
advantage of which almost any Microscope object can be successfully 
photographed. Not but that there are on the market other plates 
having qualities in every way equal to the plates particularly mentioned. 
For instance, the “Eagle” plate, sensitometer 40, is an almost exact 
duplicate of the “ Harvard,” 40, in both rapidity and relative contrast ; 
and Carbutt’s “Keystone,” sensitometer 16, is almost identical with the 
Seed, 25, in all properties except rapidity. All plates having the 
qualities of the Harvard and Eagle give great contrast and short 
range of half tones, and are therefore best adapted to objects haviDg 
but slight contrasts. With such plates satisfactory negatives can be 
made from such little contrast, that were plates like the Seed, 25, and 
Keystone, 16, used, negatives having printing contrasts could not be 
made at all. Conversely, plates like the Seed, 25, and Keystone. 16, low 
contrast and long range of half tones, will satisfactorily reproduce the 
details of objects having great density or contrast, which details would be 
entirely obliterated if plates like the Harvard or Eagle were used. 
As plates similar in other qualities often vary in rapidity, as is the case 
with the Seed, 25, and Keystone, 16, this variation can be taken 
advantage of where the light is more or less strong, or where greater 
or less rapidity is desired, without in any way affecting the result, 
so far as the printing qualities of the negative are concerned. 
I have, however, never found the most rapid plate too quick, even 
with low powers and sunlight, as I habitually use a light-filter of a 
colour complementary to that of the object photographed, For these 
filters, being generally either yellow, green, or yellowish-green, con- 
siderably lengthen the time of exposure ; so much so, that while with a 
Zeiss 2 mm. h. i. apochromatic objective, a projection eye-piece, 4, and 
an amplification of 1500 diameters, a Seed, 25, plate will require 
about 35 seconds ; a wet collodion plate, using a blue filter, would 
require but about two seconds. 
As the Seed and Harvard plates have opposite qualities, which adapt 
them to almost every object to be photographed, before using other 
makes they should be comparatively tested, either with the plates 
named, or wdth some plate with the workings of which the operator is 
familiar, when their actual qualities will be demonstrated and their 
adaptability ascertained. Only by such testing can the operator know 
exactly what to expect, or be able to arrive at the best results, for this, 
like other work connected with microscopy, should never be of a hap- 
hazard sort. 
