96 REPORT— 1861. 



produced by the Kew instrument, I commenced some preparations at my 

 own observatory for the purpose of trying whether it would be possible to 

 procure sucli pictures with my reflector. On maturing my plans I found that 

 the apparatus which it would be necessary to use would be so weighty that 

 the telescope would require to be strengthened considerably to support the 

 additional weight in the awkward position in which it would have to be placed ; 

 and it did not at first appear how this could be ronveniently done. 



Ultimately I found the means of adding a radius-bar and of supporting the 

 plate-holder, which carries a plate 18 inches square at a distance of 4> feet 

 from the eye-piece ; but here another difficulty occurred, namely, that the 

 image of the sun was so powerfully heating, that, if allowed to remain for a 

 very short time on the instantaneous slide, it heated it and ultimately set fire 

 to some part of the apparatus. A trap easy to be moved over the mouth 

 of the telescope had to be contrived, so as to open just before the instantane- 

 ous apparatus was brought into action and shut again immediately after- 

 wards. At last these mechanical difficulties were surmounted, and I 

 commenced my experiments to ascertain the best form for the secondary mag- 

 nifier : these experiments are still in progress, and some important difficulties 

 remain to be overcome before pictures of the sun's spots will be obtained 

 with that degree of sharpness which shall leave nothing to be desired. 



With an ordinary Huyghenian eye-piece, employed as a secondary magnifier 

 and placed somewhat nearer the great mirror than would be its position for the 

 most perfect optical picture, in order to throw the chemical rays further on and 

 thus bring them to focus on the plate, I have obtained some sun-pictures, of 

 very considerable promise, on the extremely large scale of the sun's diameter 

 equal to 3 feet. These pictures have only been very recently procured, and 

 I submit them to the Section because I believe that an interest is felt in the 

 progress of celestial photography, and that the Members prefer to take part 

 in the experiments, as it were, by watching their progress, rather than to 

 wait until the most perfect results have been brought about. I may state 

 that the mechanical and chemical difficulties have been surmounted, and that 

 the only outstanding one is the form of the secondary magnifier*. When this 

 has been worked out, perfect sun-pictures 3 feet in diameter will be obtain- 

 able with a telescope of 1 foot aperture, in less than the 20th of a second 

 of time. These pictures, when taken under suitable circumstances, may be 

 grouped so as to produce stereoscopic pictures, which must throw consider- 

 able light on the nature of the spots. 



It appears to me that such results must be of value to science, and that 

 the records of the state of the sun's photosphere, both as regards spots and 

 other changing phenomena, which are obtainable by means of photography, 

 are worth collecting and discussing, and that ultimately they will throw con- 

 siderable light on terrestrial meteorology. 



It is agreeable to me to work at this problem so as to point out the means 

 by which success is attainable, and I may for a time carry on the records ; 

 but it will, on reflection, be seen that these observations (if continued, as 

 they should be, for years) are likely to prove a too serious tax upon the 

 leisure and purse of a private individual. 



* Mr. Dallmeyer has lately assisted me in working out this problem, and has produced 

 already two new secondary magnifiers, each of a somewhat different construction. With their 

 aid I made a considerable step in advance, but on November 7th, 1861, was stopped by the 

 lateness of the season. 



