H. A. Rowland—Concave Gratings for Optical Purposes. 97 
Tam now making, p is about 260 inches and the width of the 
grating about 54 inches. Hence d= i00 approximately and the 
series becomes 
1 tan 4. 
1 
~ 2,000,000 
Hence the greatest departure from the theoretical ruling, even 
when tan #,=2, is 1 in 1,000,000. Now the distance apart 
of the components of the 1474 line is somewhat nearly one 
forty-thousandth of the wave-length and I scarcely suppose that 
any line has been divided by the best spectroscope in the world 
whose components are less than one-third of this distance apart. 
Hence we see that the departure of the ruling from theoretical 
perfection is of little consequence until we are able to divide 
lines twenty times as fine as the 1474 line. Even in that case, 
since the error of ruling varies as 6°, the greater portion of the 
grating would be ruled correctly. 
* T have recently discovered that each component of the D line is double prob- 
fron the partial reversal of the line as we nearly always see it in the flame 
Am. Jour, 9p emmers Series, Vou. XXVI, No. 152.—Aveust, 1883. 
