148 REPORT — 1891. 



ing telescope thereof varied rapidly with the smallest angular change of 

 its direction in spectrum place, there arose a necessity for a considerable 

 improvement of the focussing arrangement over and above what is usually 

 supplied for the visible parts of the spectrum, or had been furnished in the 

 present instance for all parts. But this improvement has now been accom- 

 plished by Messrs. T. Cooke & Sons, of York, according to a design by 

 the Secretary, enabling the focus to be set distinctly and solidly to the 

 thousandth of an inch without reference to anything but numerical tables 

 prepared beforehand and tested by photographic record. 



Again, however, in some of the most interesting of those ultra-violet 

 regions of solar spectrum light a further and more intricate difficulty of 

 a physical nature was found when photographing in the second order 

 of the Grating's spectra. For, though that operation was performed under 

 double shields of the darkest blue glass procurable, yet the red region of 

 the first order of spectrum would insist on breaking in through all ob- 

 stacles, and showing itself even brilliantly by means of the anomalous ultra- 

 red ray transmitted by the supposed most pure and densely blue, or violet, 

 glass known ! One possible method of getting rid of this difficulty imme- 

 diately seemed to be by photographing only in the first order of the 

 Grating's spectrums, throughout whose violet fields there is no red band 

 of any other order to come in — blue glass in place or not. But could 

 sufficient spectrum separation of lines be thereby obtained, and without 

 any other drawback ? 



To meet this essential problem Messrs. T. Cooke & Sons, of York, 

 were again applied to, and they constructed within the grant made to 

 the Committee an extra-large Barlow photo-achrom-concave lens, which 

 magnified the previous image of the inspecting telescope's object-glass 

 by 2*3 times, or rather more than the first order of the Grating's spec- 

 trums is magnified, in separation only, by the second order. And if by 

 the Barlow concave the magnifying is both in separation and in height 

 of lines (and therefore weakening to the intensity of the image), it was 

 hoped that longer exposures could be freely given. So that then, with 

 them, would come the final trial, which has still to be made — whether the 

 exquisite definition of the first order of spectrum cannot be lenticularly 

 magnified to the required degree, with less loss of that still more valuable 

 feature, definition, than what takes place when it is diffractionally magni- 

 fied (at least in the Secretary's Grating spectroscope) by resorting to ita 

 second order of spectrum ? 



This is the main point, then, up to which the Secretary's research has 

 just arrived by aid of the British Association's grant of 1890. For while 

 the whole of that sum has now been expended on the above-mentioned 

 major subjects and a number of minor improvements and working particu- 

 lars bearing on the same ends, and nothing further in the way of grant 

 is now being asked for, it leaves sufficient material in Dr. C. Piazzi 

 Smyth's hands for much work in the months to come. In earnest whereof 

 he begs to send some of his accomplished work during the last nine 

 months, in the shape of two album cases, each containing twenty-six of 

 his separately mounted and scaled but continuous solar spectrum mag- 

 nified photographs of lines in the violet and ultra-violet, besides a third 

 and thinner album case of previously taken eye-and-hand-made drawings 

 at the same instrument, but of the easier half only of the same subjects, 

 for inter-comparison of the two methods which are past, and in prepara- 

 tion for the third, which is to come. 



