19'^] FOR A CONCAVE GRATING. 561 



camera may be connected by a light-proof bellows or other en- 

 closure, so that the instrument may be used in an undarkened room. 

 This bellows may be supported partly upon the blocks which carry 

 the linkage. 



The only part of the construction that may seem to ofifer dififi- 

 culty is in making the linkage true, but this should not prove a seri- 

 ous obstacle. All that is necessary is that the four arms of each 

 parallelogram shall be of equal length and that there shall be no 

 play at the joints, and it should easily be possible to do this with 

 sufficient accuracy. It may be added that the linkage, though ex- 

 tremely convenient, is not essential. The grating may be turned by 

 hand to the angle corresponding to the wave-length desired, and 

 then moved along the track until the light is focused on the slit. 

 The camera may then be rotated until the spectrum is in focus.^^ In 

 this way it may be possible to use gratings of much larger radius, 

 and so avoid the errors produced in ruling on a surface of too great 

 curvature. 



The great compactness of the mounting makes it available for 

 use in astronomical spectroscopy, from which the concave grating is 

 practically barred when the Rowland mounting is used. The in- 

 strument may be mounted upon a telescope in the prolongation of 

 its axis so that the slit lies in the focal plane of the objective. In 

 the case of a star image the slit could be dispensed with, and the 

 astigmatism of the grating would produce a spectrum of finite 

 width. A more rigid and more convenient arrangement would be 

 to mount the guides for the grating upon the tube of the telescope 

 on the side opposite to the declination axis. The light could be 

 brought to a focus by the objective at the side of the field nearest 

 the slit and thrown upon the slit by totally reflecting prisms. No 

 harm would be done by any possible astigmatism which would 

 merely be added to that of the grating, and slight irregularities in 

 driving would be equally harmless. 



It remains to consider the character of the spectra produced. 

 The chief advantages of Rowland's mounting are that the spectrum 

 is normal and always of the same scale, and that the plate is per- 



"This is in fact the arrangement described by Eagle (I. c). 



