1891.] MICROSCOPICAL JOURNAL. 171 



page of development before completion will produce erroneous results. 

 The illustration given with this article is a reproduction of the result 

 arrived at by comparing a " Harvard" plate, sensitometer 40, with a 

 " Seed" plate, sensitometer 25, in the manner above described. It is a 

 reproduction of the negatives themselves (not a print from them), so 

 the lighter bands represent the thinner bands of the original negatives. 



The great difference in the two negatives is seen at a glance. The 

 greater rapidity of " Seed " plate is shown by band 9 in the plate, 

 where the light had to act through nine thicknesses of paper before 

 acting upon the plate, being equally as dark as band 5 in the " Har- 

 vard," where the light had to act through but five thicknesses. The 

 comparative rapidity of the Seed to the Harvard is therefore as nine to 

 five ; or for practical purposes it may be considered as double. The 

 greater contrast of the Harvard, and longer range of half-tones of the 

 Seed are shown by the same range being gone through in five bands in 

 the Harvard, i. e., from band i to 5, that requires nine bands in the 

 Seed, i. e., from band i to 9. In other words, a certain gradation of 

 light in an object photographed, which will give with a Seed plate a 

 certain contrast in the negative, will with a Harvard plate give practi- 

 cally 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 de- 

 sired. Also, 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 ad- 

 vantage of which almost any microscopic object can be successfully 

 photographed. Not but that there are on the market other plates hav- 

 ing 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 having but slight 

 contrasts. With such plates satisfactory negatives can be made from 

 such little contrast, that were plates like the Seed, 25, or Keystone, 16, 

 used, negatives having printing contrast could not be made at all. Con- 

 versely, plates having like the Seed, 25, and Keystone, 16, low con- 

 trast and long range of half-tones, will satisfactorily reproduce the de- 

 tails 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, t6, this variation can be taken ad- 

 vantage of where the light is more or less strong, or where greater or 

 less rapidity is desired, without in any way afiecting 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 



