MEMOIR VII. ] STUDIES IN THE DIFFRACTION SPECTRUM. 125 



but these it is not necessary to consider. The two dark 

 spaces, and especially the first, are the objects mainly to 

 be examined. 



Previously to 1844 I had attempted to obtain diffrac- 

 tion photographs with the grating that Mr. Saxton gave 

 me, and had met with great success. In that year I 

 published engravings of them, the originals having been 

 made on silver daguerreotype tablets, in use at that time. 

 By these I carried spectrum impressions as far as the 

 wave-length 3800, and therefore encroached considerably 

 on the dark space p, towards A'. But collodion, since 

 introduced, is a much more sensitive preparation. It 

 has enabled Henry Draper, who has produced superb 

 photographs of the more refrangible regions, to carry 

 the impressions as far as 3032. 



According to M. Mascart, waves are emitted by incan- 

 descent cadmium having a length not exceeding 2200. 

 These stand still further in the dark space p. 



In these excursions into the dark space the experi- 

 ments of Professor Stokes on the long spectrum of elec- 

 tric light become not only interesting, but very impor- 

 tant ; for as we gradually approach A', the wave-length 

 of the incident radiation is continually diminishing, and 

 at A 7 it becomes zero. That point is the supreme limit, 

 beyond which no radiant manifestation of any kind is 

 possible. 



The goal towards which experimental investigation is 

 tending is therefore obvious. We are gradually groping 

 the way across the dark space, and expect one day to 

 reach the bright streak that lies at its terminus. At 

 every step of advance the ether waves are becoming 

 shorter and shorter, and the vibrations more and more 

 rapid. When the journey is accomplished, a region will 

 have been gained in which the waves are infinitely short, 

 and the vibrations infinitely rapid. 



