A TTT 
0. N. Rood on Diffraction with the Compound Microscope, 329 
tain number of prismatic spectra on either side, the purity of the 
colors being dependent on the regularity with which the lines 
areruled. With lines ;4,th of an inch apart, five spectra on 
either side were observed. If however the cornea of a fly be 
taken as a grating, and viewed at 11 inches, a central white spot 
is seen, surrounded on all sides with great numbers of prismatic 
The appearance is one of great beauty. 
Having thus described an easy method of performing expeti- 
ments on diffraction, we proceed to notice one or two of its bear- 
ings On Microscopic vision. And we would state in the outset, 
that all the experiments alluded to above may be performed by 
the microscope when arranged as it ordinarily is for observation ; 
that is, let an inch or a half inch objective be screwed on, th 
eye-piece being in its proper position; let parallel rays of the sun 
be thrown up through the body by the plane mirror, and let an 
aperture > ;',,th or ;,';,th of an inch be placed on the stage and 
viewed as an ordinary object. As long as the aperture is in focus, 
nothing remarkable will be observed, but if the compound body 
carried within or without the focus, the aperture will be seen 
to dilate and to become surrounded with colored rings precisely 
as above, Reckoning from the centre, the rings wil be yellow, 
dark red, blue, yellow, red, blue, é&c. If a fine slit be used in- 
 -aG, the same sets of colored bands are seen as described above ; 
Meither case, as the object lens approaches the focal point the 
Hngs or bands contract and disappear. By turning the light a 
little obliquely, so as to make it faint, or by using a larger aper- 
ture with rather faint sunlight, dark rings or lines can be observed 
Mn the centre of the white spot. If the edge of an opaque body 
“© drought into the field of sunlight, and if it is a little out of 
the focus, it will be seen bordered with the same dark or colored 
Nes as described above. ‘To observe the internal fringes let a 
Piece of fine wire, (-1,th of an inch,) be placed on the stage, 
200 
and viewed a little out of the focus, it will be seen filled up in- 
ffom the centre, they will be, blue, yellow, red, blue, &c., this 
: case when direct ( 
Phenomena being different with oblique hight. 
_ We have now seen that when the direct rays of the sun, or a 
observed that when f even 200 diameters, the ob- 
- ed tha under a power of even ; rs, 
Het being most accurately aed. it exhibits marked phenomena 
