278 Transactions of the Society. 



enable photography to do for the Microscope what it has already 

 done for the telescope, and reveal detail beyond the range of 

 vision ; and this is the more feasible since in the arc we possess, 

 as I have shown, a powerful and easily controlled source of them. 



It is stated by Stokes that the ultra-violet spectrum of the arc 

 light is with quartz lenses and prisms six times as long as the 

 visible spectrum ; and all of this can be photographed. With glass 

 prisms it is not of course so long, nor so intense ; and for micro- 

 scopical purposes it is chiefly glass which we must consider. But, 

 according to Abney, much more may be done with glass than is 

 generally supposed, and " with pure white flint-glass prisms the 

 furthest lines in the solar spectrum can be photographed." This 

 ■was done by Cornu, using crown and flint-glass prisms, as far as 

 the Fraimhofer line U. The wave-length of this line in ten-millionth s 

 of a millimetre by Cornu's map is 2948, that of D l3 the line of 

 maximum visibility, being 5889. The wave-length of U is, there- 

 fore, almost exactly one-half that of D, and should give twice the 

 resolution. But Cornu, as stated above, used prisms and lenses 

 of crown and flint. The arc light moreover extends a great deal 

 further into the ultra-violet than does sunlight, so that by the 

 proper choice of glasses (of which so many are now available) and 

 of other media, much more might be done. Fluorite is fortunately 

 very transparent to these rays, as much so, according to Miller's 

 tables, as quartz. If opticians could thus provide us with lenses 

 transmitting rays far into the ultra-violet, and at the same time 

 fully corrected for them, we might hope to achieve a considerable 

 advance. 



I find that a lens of the kind suggested was actually made by 

 Butherfurd for the telescope which he used in stellar photography. 

 " Mr. Butherfurd," says Bockyer, " simply discards the visual rays, 

 and brings together the violet ones ; the result of his work being 

 a telescope through which it is impossible to see anything, but 

 through which the minutest star down to the tenth magnitude 

 can be photographed with the most perfect sharpness. This is 

 the instrument of the future, so far as stellar photography is 

 concerned." 



The two kinds of work are not of course quite analogous. 

 The additional detail is brought out in telescopic work mainly by 

 prolonged exposure, and Butherfurd did not apparently take the 

 ultra-violet into consideration ; yet one may venture to predict 

 that the photographic objective of the future will be something 

 of the same kind. 



The difficulty of constructing such an objective would no doubt 

 be great, and its use when constructed would be far from easy, 

 especially in focussing ; but this might not be so difficult if it were 

 corrected so as to bring the visible violet to the same focal point 

 as the ultra-violet, since the focussing on the coarser detail might 



