330 O.N. Rood on Diffraction with the Compound Microscope. 
of diffraction. Let us now take one of the simplest cases of 
bodies viewed by oblique light, viz. a single fibre of black silk, a 
single strand of a spider’s web, or a fine scratch in glass ; let it be 
viewed under a power of 400 diameters, the light being that of 
the sun, thrown obliquely across the object most aceurately fo- 
cused. The object will be seen to dilate, and three bands of 
color, blue, red and yellow, will represent it; the colors all broad 
and distinct. Within the bands and running parallel with them, 
are great numbers of fine dark lines, placed very near to each 
other, and with éonsiderable regularity, and in fact closely resem- 
bling the lines on certain test objects; moreover, it would seem 
that the better the object lens, the more distinct the lines beeome. 
That these black lines are only fringes of diffraction may be 
readily shown, for by moving the compound body a little out of 
focus, they develop into the ordinary colored fringes. If the 
line be a strand from the spider’s web, the fine black lines are 
very abundant, and extend on one side a considerable distance 
beyond the colored space. If a fine scratch on glass be view 
in this manner, under 400 diameters, the light can be so arranged 
that it shall exhibit the same phenomena; in general, if the hne 
has any breadth, and if it be rotated so that the light is thrown 
along its length, a position will ordinarily be found when the ex- 
ternal fringes for the most part disappear, while the internal ones, 
if there are any, become more distinct. The foregoing may per 
haps throw some light on the fact that so many persons see 
gitudinal lines on test objects, (Navieule,) while they fail to 
show the transverse ; but nevertheless, in using very oblique sun- 
light not only is a single line always accompanied by many par- 
allel fringes of diffraction, but after these have been made to dis- 
appear by revolving the line, often a distinct set of ee 
t wo 
In viewing fine gratings under the microscope, that species of 
illumination called background should be employed, and m , 
of an artificial grating which could not readily be procured sul- 
Tt will be noticed that when the light is thus thrown along the 
lines, the scale is generally of a dull blue; on revolving it — 
Bg from blue to bright red, to orange, to yellow: ” 
the next stage it is very bright and slightly tinged with ve 
turning it farther, it changes from yellow to red, to the origi! f 
dark blue. When it has reached its maximum of brightness 
d bod: 
beautiful prismati 
scale, consisting of red, orange, yello n and blue. 
jth ange, ye ow, green = . 
scales of the Lepisma, with the one inch objective, show — 
