74 EEPOET — 1892. 



Second Report of the Committee, consisting of Professor Liveing^ 

 Dr. C. PiAzzi Smyth (Secretary), and Professors Dewar and 

 ScHUSTEE, appointed to co-operate tvith Dr. C. Piazzi Smyth in his 

 researches on the Ultra-violet Rays of the Solar Spectrum. 



The present report is on the proposed experiments (from September, 

 1891, to January, 1892) for enabling Dr. C. Piazzi Smyth to improve 

 certain points in the taking of his solar-spectrum photographs in the 

 ultra-violet by aid of additions to the apparatus obtained through means 

 of a grant from the British Association at Leeds in 1890. 



The Report continuates the last one by the same committee, as 

 printed in the British Association's Cardiff volume of 1891, at pp. 147 

 and 148 thereof, said space being then taken up with little more than 

 descriptions of vrhat the apparatus, then only just finished, was intended 

 for. Now, however, a sufficient amount of experiments have been obtained 

 to allow the results to be classified and collated under three several 

 heads, or thus : — 



(1) Improved focussing means for setting the focus of the viewing, 

 or photographic telescope, both more accurately and easily as well, 

 from previous book-record, rather than from renewed eye-and-hand 

 observation on every occasion. This was carried out mainly and success- 

 fully by supplying wheels ten inches in diameter, and nicely graduated 

 on their circumferences, to either end of the ordinary axle of pinion- 

 movement of the focussing tube, taking care also to turn the said pinion 

 at the last moment in the direction of increasing the readings and noting 

 what they were. This record method of focussing, too, it is believed, is 

 one which will be found of very general application, and much used every 

 coming year, now that photography is continually substituting more and 

 more the observer's eye and hand, with almost all kinds of optical notation 

 of luminous phenomena. 



(2) Improved magnifying means were next required for the viewing, 

 and equally photographing, telescope. The chief feature necessary here 

 was a large field with the increased magnifying power, and was given to 

 a considerable extent by a grand Barlow-achromatic concave lens placed 

 inside the usual telescope tube, by Messrs. T. Cooke & Sons, of York. 



For mere magnifying, however, wherever the part of the spectrum 

 under examination permits it without other addition, I have since then fully 

 made up my mind that the second order of Professor Rowland's later and 

 nnprecedentedly fine Gratings from his new ruling engine, give sharper 

 magnifying to the spectrum than any lens I have experimented with. 



But they give it in a different way, i.e. the second orders of Grating's 

 spectra do ; for they magnify only in one direction — that of separation — 

 while a lens magnifies in a direction at right angles to that also. That 

 feature is no doubt so much the worse for the lens, because it weakens 

 the intensity of a continuous spectrum operated upon by it. But then 

 there is another feature which is bad for the second, or any subsequently 

 still more magnified, spectrum-order of a Grating, viz. that they admit 

 the red light of a previous order in the middle of their own violet ; unless 

 some possibly very absorptive liquid be employed to stop such red light 

 where it is not wanted. 



