Michelson — Spectroscope without Prisms or Gratings. 215 



Art. XXIX. — A Spectroscope without Prisms or Gratings / 

 by A. A. Michelson. 



The resolving power of a diffraction grating is proportional 

 to the product of the total number of lines by the order of the 

 spectrum observed. But little effort seems to have been made 

 to make a decided step in the direction of increasing the order 

 of the spectrum observed, and this is doubtless because for a 

 grating acting by opacity the brightness of the spectrum 

 diminishes very rapidly as the order increases. This difficulty 

 has been successfully overcome by ruling the lines in such a 

 way as to concentrate the greater proportion of light in one 

 spectrum, but so far as I am aware such attempts have been 

 limited to the first, second or third spectrum and the results 

 even here are somewhat fortuitous. 



It seems nevertheless quite possible to construct gratings 

 which shall throw a quite large proportion of the light in very 

 high orders of spectra — say the hundredth — in which case the 

 grating space must be of the order of a hundred waves or say 

 twenty to the millimeter, instead of a thousand. The lines 

 would have to be drawn with no more accuracy. than before, 

 and the grating could be completed in a very short time and 

 temperature changes would have a much smaller effect than at 

 present. 



It may be that there are more serious practical difficulties in 

 the way of such a ruling as is represented in fig. 1 than would 



be anticipated. Especially may this be true if the greater part 

 of the light is to be returned in the direction from which it 



