112 PROCEEDINGS OF THE AMERICAN ACADEMY. 



There is, however, the rather serious inconvenience that the examina- 

 tion of a single line requires a considerable time, often several minutes, 

 and during this time the character of the radiations themselves may be 

 changino;. 



Besides this, nothing can be determined regarding the nature of these 

 radiations until the " visibility curve " is complete, and analyzed either 

 by calculation or by an equivalent mechanical operation. 



Notwithstanding these difficulties, it was possible to obtain a number 

 of rather interesting results, such as the doubling or the tripling of the 

 central line of Zeeman's triplet, and the resolution of the lateral lines 

 into multiple lines ; also the resolution of the majority of the spectral 

 lines examined into more or less complex groups ; the observation of 

 the effects of temperature and pressure on the width of the lines, etc. 



It is none the less evident that the inconveniences of this process are 

 so serious that a return to the spectroscopic methods would be desirable 

 if it were possible, 1st, to increase the resolving power of our gratings ; 

 2d, to concentrate all the light in one spectrum. 



It is well known that the resolving power of a grating is measured by 

 the product of the number of lines by the order of ihe spectrum. Atten- 

 tion has hitherto been confined almost exclusively to the first of these 

 factors, and in the large six-inch grating of Professor Rowland there are 

 about one hundred thousand lines. It is possible that the limit in this 

 direction has already been reached; for it appears that gratings ruled 

 on the same engine with but half as many lines have almost the same 

 resolving power as the larger ones. This must be due to the errors in 

 spacing of the lines ; and if this error could be overcome, the resolving 

 power could be augmented indefinitely. 



In the hope of accomplishing something in this direction, together with 

 Mr. S. W. Stratton, I constructed a ruling engine in which I make use 

 of the principle of the interferometer in order to correct the screw by 

 means of light-waves from a homogeneous source. This instrument 

 (only a small model of a larger one now under construction) has already 

 furnished rather good gratings of two inches ruled surface, and it seems 

 not unreasonable to hope for a twelve-inch grating with almost theoreti- 

 cally accurate rulings. 



As regards the second factor, the order of the spectrum observed, but 

 little use is made of orders higher than the fourth, chiefly on account 

 of the faintness of the light. It is true that occasionally a grating is 

 ruled which gives exceptionally bright spectra of the. second, or third 

 order, and such gratings are as valuable as they are rare ; for it appears 



