Ox CONCAVE GRATINGS FOE OPTICAL PURPOSES 499 



As the radius of curvature of concave gratings is usually great, the 

 distance through which the spectrum remains practically normal is very 

 great. In the instrument which I principally use, the radius of curva- 

 ture p, is about 21 feet 4 inches, the width of the ruling "being about 5-5 

 inches. In such an instrument the spectrum thrown on a flat plate is 

 normal within about 1 part in 1,000,000, for 6 inches and less than 1 in 

 35,000, for 18 inches. In photographing the spectrum on a flat plate, 

 the definition is excellent for 12 inches, and by use of a plate bent to 11 

 feet radius, a plate of 20 inches in length is in perfect focus and the 

 spectrum still so nearly normal as to have its error neglected for most 

 purposes. 



Another important property of the concave grating is that all the 

 superimposed spectra are in focus at the same point, and so by micro- 

 metric measurements the relative wave-lengths are readily determined. 

 Hence, knowing the absolute wave-length of one line, the whole spec- 

 trum can be measured. Professor Peirce has determined the absolute 

 wave-length of one line with great care and I am now measuring the 

 coincidences. This method is greatly more accurate than any hitherto 

 known, as by a mere eye inspection, the relative wave-length can often 

 be judged to 1 part in 20,000 and with a micrometer to 1 in 1,000,000. 

 Again, in dealing with the invisible portion of the spectrum, the focus 

 can be obtained by examining the superimposed spectrum. Captain 

 Abney, by using a concave mirror in the place of telescopes, has been 

 enabled to use this method f or^ obtaining the focus in photographing the 

 ultra red rays of the spectrum. It is also to be noted that this theorem 

 of the normal spectrum applies also to the flat grating used with tele- 

 scopes and to either reflecting or transmitting gratings; but in these 

 cases only a small portion of the spectrum can be used, as no lens can 

 be made perfectly achromatic. And so, as the distance of the microme- 

 ter has constantly to be changed when one passes along the spectrum, 

 its constant does not remain constant but varies in an irregular man- 

 ner. But it would be possible to fix the grating, one objective and the 

 camera rigidly on a bar, and then focus by moving the slit or the other 

 objective. In this case the spectrum would be rigidly normal, but 

 would probably be in focus for only a small length and the adjustment 

 of the focus would not be automatic. 



But nothing can exceed the beauty and simplicity of the concave gra j - 

 ing when mounted on a movable bar such as I have described and illus- 

 trated in Fig. 1. Having selected the grating which we wish to use, 

 we mount it in its plate-holder and put the proper collimating eye-piece 



