28 riiK Ausoni'i'iON and kmission ok aiij. 



treated with ilie must scrupulous care and the material is faultlessly tiglit, small 

 amounts of absorljed gas little by little get set free. In the same way, some air 

 diffuses in through the grease film as soon as it has been for some time in use ; and 

 the inflow becomes continually greater the longer the apparatus is left to itself 

 without being pumped out. 



All this shows how little any apparatus without such a slide or other equiva- 

 lent contrivance can be recommended for the investigation of the ultra-violet. 



The arrangement of the slide is founded on the supposition that only uncurved 

 plates of moderate length are to be used. It would be desirable, howevei", for a 

 better ti^eneral view of the whole i-egiou to be photographed as well its for the sake 

 of savin" some of the time lost during exposures, to be able to photograph at a 

 single exposure a greater extent of the specti'um, if possible, the entire sensitive 

 i-eo-ion of the plate. In this way, we might pei-haps photograph at once the whole 

 specti-um from the cyan blue to wave-length 100 /</< or indeed still farther. The 

 great curvature of the photographic plate whicli would be lec^uisite could be ob- 

 tained with ordinary plates, as is well enough known, by means of a film oi- of 

 mica. Not so with ultra-violet plates, since the preparation of their coating is 

 creatly affected l)y any unevenness of the surface to be coated, from w^hich films 

 are far from free and even sheets of mica are not sufficiently so ; for the coating is de- 

 posited thicker in depressions and thinner on elevations of the surface, giving rise 

 to differences of sensitiveness as well as to other defects.' Even if one is willing 

 to overlook these inconveniences and to content himself with graphically less 

 perfect spectrograms, or if the process of preparation of ultra-violet plates could be 

 improved in this respect, the inconvenience of the varying activity of the spectrum 

 in different spectral regions will still subsist. Judging by my prismatic photo- 

 graphs tliis variation is gi-eatest in the ultra-violet, that is, precisely in the pai't 

 which requires to be photographed in vacuo. The consequence would be that the 

 most active regions would be greatly over-exposed before the weaker parts be- 

 came developable at all, and as a result partial photographs would again have to 

 be reverted to. I cannot therefore advise the employment of long plates, par- 

 ticularly since, for constructive i-easons, the advantages of the slide would thus 

 disappear, and at eveiy change of plates the spectrogi'aph would fill with air and 

 would have to be pumped out. In like manner the employment of gratings of 

 long radius is not unhesitatingly to be adopted where the ultiaviolet is to be 

 observed to its most refrangible rays. For there is here the unavoidable difficulty 

 of absorption, however far the exhaustion be earned, owing to the small residue of 

 air as well as the vapors from the airpuinp ; the absorption, of course, being 

 nuich greater with the long distance which the rays have to traverse owing to the 

 long i-adius of the giating. This obstacle of absoi-ptiou is the reason why the 

 shortest wave-lengths are incomparably more easily investigated with short foci 

 than with larger apparatus. 



A it: — I must pass ovei- my observations of the emi.ssion-s[)ectrum of air, on 



' V. Schumann : On an improved method for the preparation of •ultra-violet plates. Annalen 

 Jer Physik, vol. v, pp. 349-374, 1901. 



