a a 
0. N. Rood on Diffraction with the Compound Microscope. 331 
similar spectra side by side, others three ; others again, one prin- 
cipal spectrum near the spot where the image of the scale van- 
ished, and another faint spectrum which advances or recedes from 
the principal one, as the compound body is moved up and down, 
The distinctness of the colors in these spectra, depending as they 
do on the regularity with which the lines are drawn, furnish a 
means of forming a judgment on this point. The spectra are of 
couse formed at right angles to the direction of the lines. By 
means of these spectra the existence of lines on certain scales was 
determined was quite coarse—could readily be seen by the naked 
“Y¢; by using a fine line in the micrometer eye-piece, it is evi- 
deut that this method admits of being carried much farther. 
In conclusion, we would remark that this subject seems worthy 
the ic 
“opie obj cts, but also because it is probable that by a skillful 
Use of Dishes at ‘diffraction, we should be able to detect, in- 
ditectly, the existence of certain kinds of structure, when th 
Dest objectives had wholly failed to demonstrate it. 
