Wave-length of Light. 279 



grating that is thus affected, in which case the error may be 

 due to a change of temperature or to a fault in the screw. If 

 an error of this kind is extensive, it will produce the effect of 

 two contiguous gratings of different grating-space, injuring 

 the definition and widening or reduplicating the lines. When, 

 however, the abnormal spacing is confined to a few hundred 

 lines, it produces no visible effect when the whole grating is 

 used, but simply diffuses a small portion of the light, and 

 increases or decreases the average grating- space. For it is 

 evident that such a portion of the grating must possess little 

 brilliancy and less resolving power ; and the more its spacing 

 differs from that of the rest of the grating, the less chance of 

 visible effect and the greater error introduced. Such a fault 

 is compatible with the sharpest definition, but can be detected 

 by cutting down the aperture of the grating till the spectrum 

 from the abnormal portion is relatively bright and distinct 

 enough to be seen. The effective grating-space, producing 

 the spectra on which measurements are made, is, of course, 

 that of the normal portion only. Both the gratings used in 

 these experiments were affected by the above error, No. I. 

 very slightly, No. II. somewhat more seriously. Not only 

 the discrepancies between different gratings, but those between 

 different orders of spectra in the same grating are due to this 

 cause. For while in one order, where the effect due to the 

 abnormal portion is imperceptible, the spectrum as measured 

 is produced by the effective grating-space alone, in another 

 order there may be produced a slight shading-off of the lines, 

 so that their apparent centres may correspond approximately 

 to the average grating-space. In any case, it is quite clear 

 that a combination of the results from different orders of 

 spectra will not eliminate the error. 



The remedy lies either in stopping out the imperfect por- 

 tion of the grating, or measuring it and introducing a cor- 

 rection. As the work of angular measurements was nearly 

 finished before the study of the gratings was begun, owing 

 to a delay in getting apparatus, the latter course was adopted 

 in these experiments. Each grating was examined in detail, 

 and the relation of the grating-spaces in the various portions 

 of it carefully determined. From these data a simple gra- 

 phical method gave the correction to be applied to the wave- 

 length. In each grating the fault was confined to a small 

 portion ; and as the order of the spectrum employed in each 

 was selected on account of its good definition and freedom 

 from anything like haziness or shading-off of the lines, it seems 

 safe to assume that the abnormal portion produced no visible 

 effect, and that, consequently, the correction above mentioned 



