in Prof. Rowland's Concave Gratings. 197 



144 # 8, than is the case with the other grating. The error 

 necessary to produce such a difference does not exceed qqqq 

 of the distance to be determined. ^ 



On calculating as before the quotients — , we obtain 



^^=22440 + 120. 



The radius of curvature of this grating was found to be 40*3 

 millim. greater than that of the other, viz., 6474 millim., the 

 uncertainty amounting to about 2 millim. With this value 

 of p and the nominal value of co, we find 6 = 30' 16" and 

 00! = 57-0 millim. 



To judge from the inscriptions on the gratings the obli- 

 quity ought to have the same direction in both cases. 



V. The focal curve that passes through the centre of cur- 

 vature of the grating being a circle, as shown above, it will 

 always be possible to adjust the spectroscope in such a 

 manner as to obtain in all positions distinct images of the 

 spectra without altering the distance between the grating and 

 the eyepiece. It follows from the preceding that this is 

 effected by causing the girder to act the part of the diameter 

 OiC of the true focal circle instead of that of a radius of 

 curvature of the grating. Hence, in the first grating the 

 girder ought to be lengthened by 0*17 millim., then turned 

 round the point C through an angle S = 24'47". The easiest 

 way of doing this is to displace the cross hairs of the eyepiece 

 in O with the micrometer through a distance equal to the 

 corresponding arc 46*4 millim. (to left or to right according 

 to the side of the grating which we wish to make use of), 

 and afterwards turning the grating until the cross hairs and 

 their image coincide again. Then in all positions of the 

 girder the slit ought to remain on the true focal circle. The 

 exactitude of the theory and the measurements is confirmed 

 by the fact, that after the execution of these adjustments the 

 image became perfectly defined through the whole spectrum. 



It would also be possible to obtain distinct images in all 

 positions by another method, viz., by altering the angle of the 

 rails by a quantity 8. In that case the centre of curvature 

 ought not to be displaced, but it is necessary to turn the 

 micrometer and the photographic box through an angle 8, 

 so that they may be always tangents of the focal circle. 



However, the supposition which we have made, that the 

 true focal circle passes through the apex C of the grating, 

 does not possess any very high degree of probability, because 

 in that case the curve would intersect the surface of the 



