42 JOURNAL OF PROCEEDINGS. 
tensest darkness, a bright spot—as bright, indeed, as though 
the disc were absent. (Arago’s experiment.) This result 
has been obtained and photographed at the University of To- 
ronto with a disc 3% inches in diameter, though in Arago’s 
original experiment the diameter was 1-12 of an inch. 
If the light pass by a narrow straight obstacle (such as 4 
needle), or through a small opening, similar bending effects, 
often exquisitely beautiful, are obtained. These bending ef- 
fects are due to diffraction. 
The most important example of diffraction is in the grat- 
ing. A grating consists of lines, usually very straight, very 
equally spaced and very numerous, scratched on glass or on a 
polished metallic surface. The scratch is opaque (or non-re- 
flecting) and the interspace is transparent (or reflecting). 
When light passes through (or is reflected from) a grating it 
is all broken up, and when re-united on a screen gives beautiful 
spectra, which, however, are distinguished from spectra pro- 
duced by a prism in that the red is deviated most and that the 
colors are spaced in a manner directly variable with their wave- 
lengths. On account of its uniform nature the spectrum is 
called normal, and many important investigations have been 
made by it—pre-eminently by the late Prof. Rowland, of Balti- 
more, who devised an almost perfect machine to rule his 
eratings. 
If we know the number of lines per inch (from 1 to 20 
thousand), and can measure the angle the light is deviated 
from its original direction, the wave-length can be determined 
by a very simple formula. 
The best measurements are made on a spectrometer, which 
“consists of a graduated circle carrying a collimating and an 
observing telescope; but very fair results can be obtained with 
extremely modest apparatus. | 
Using a grating (made by photographing from the origi- 
nal, just as a lantern slide is made from a negative by contact), 
with 3,000 lines to the inch, and a simple paper scale, the wave- 
length of the yellow light emitted by a Bunsen burner, when 
salt is thrown into it, was found with considerable accuracy. 
Its true length is 0.000.589 millimetre, or 0.000.023 inches. 
