ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
655 
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 but nine thicknesses of paper before acting 
upon the plate, being equally as dark as band 5 in the “ Harvard ” 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 
Fig. 73. 
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 1 to 5, that requires nine bands in the Seed, i. e. from band 1 to 
9. In other words, a certain gradation of light in an object photo- 
