BELL. — OPACITY OF GLASSES FOR THE ULTRA-VIOLET. G77 



Figure 1 shows the spectrum of the bare tube under these new con- 

 ditions at the same current of 4 amperes used in all the spectrograms, 

 and needs no further comment. 



Figure 2, Plate 2, is an interesting illustration of the powerful 

 absorption produced by a modern anastigmat lens in the ultra-violet 

 portion of the spectrum. It was obtained merely by substituting the 

 photographic lens for the quartz lens previously used in focussing the 

 tube upon the slit. The particular lens used was a Zeiss Unar, series 

 1 B No. 5, of 155 mm. equivalent focus. This lens is composed of 4 

 thin separate elements, having an aggregate mean thickness of between 

 10 and 11 mm. It will be noted that the absorption for wave lengths 

 less than 365 yx/x is very active, and that all the ultra-violet lines are 

 somewhat weakened. The practical significance of the spectrogram is 

 that even the first-class modern photographic lens is practically almost 

 opaque to wave lengths below 365 /x/x, and it points out the necessity 

 of using special glasses and special constructions for spectrographic or 

 astrographic work. For there is ample intensity in the ultra-violet spec- 

 trum of daylight to give trouble were the lens fully transparent down 

 to the limit of the rays ordinarily transmitted by the atmosphere. As 

 Zschimmer has shown,^ one can work clear down to this limit with the 

 Uviol glasses, which of course should be achromatized with this point 

 in view. The Unar lens, like some others of recent type with separate 

 lenses, suffers more from loss of light by reflections at the multiple 

 surfaces than would the older anastigmats with thick cemented lenses, 

 but probably much more than makes up for this loss by the lessened 

 thickness of glass. The loss from reflections is quite a serious matter, 

 inasmuch as the refractive index of an ordinary crown at say w. 1. 

 350 fifx. to 340 fifi is in the vicinity of 1.55 and that of ordinary flints is 



about 1.65. Reckoning the losses by Fresnel's formula, ( J , with- 



out applying additive corrections for the obliquity of incidence, it ap- 

 pears that in a 4-lens separated system the loss of light by reflections will 

 amount to nearly 35 per cent in this region of the spectrum ; and assum- 

 ing a thickness of 10 mm. for the crowns and an equal amount for the 

 flints, the absorptive loss for the former can scarcely be less than 20 per 

 cent and that in the latter scarcely less than 70 per cent, so that on the 

 whole hardly 20 per cent of the incident light would reach the plate. Evi- 

 dently the increased absorption due to the greater thickness of glass 

 would more than overbalance the gain from reduced reflections in the 

 case of anastigmat lenses of the common symmetrical cemented type in 



* Loc. cit. 



