ON THE PROGRESS OP CELESTIAL PHOTOGRAPHY. 95 



The next subject to which I have to call your attention is the photographic 

 depiction of groups of stars — for example, such as form a constellation like 

 Orion, — in other words, the mapping down the stars by means of photography. 

 I have made several experiments in this direction, and have obtained satis- 

 factory results, and I believe that at last I have hit upon an expedient 

 which will render this method of mapping stars easy of accomplishment. 

 The instrument best adapted for this object is a camera of short focal length 

 compared with its aperture, Jike the ordinary portrait-camera, — the size of the 

 lens being selected to suit the scale of the intended photographic map, and the 

 camera, of course, mounted on an equatorial stand with a clock-work motion. 



The fixed stars depict themselves with great rapidity on a collodion- 

 plate; and I have experienced no difficulty in obtaining pictures of the 

 Pleiades by a moderate exposure even in the focus of my telescope ; they 

 would be fixed much more rapidly by a portrait-camera. The difficulty 

 in star-mapping does not consist in the difficulty of fixing the images of 

 stars, but in finding the images when they are imprinted ; for they are no 

 bigger than the specks common to the best collodion. It is of no service 

 attempting to overcome the difficulty by enlarging the whole picture ; but 

 something may be done by causing the images of the stars, which are 

 mere spots, to spread out into a cone of rays by putting the image out of 

 focus and thus imprinting a disc on the plate instead of a point. Last year 

 was so fully employed that I have not yet had time to develope fully this 

 method, but I have ascertained its practicability. 



Some curiosity naturally exists as to the possibility of applying photo- 

 graphy to the depiction of those wonderful bodies the comets, which arrive 

 generally without anything being known of their previous history and abso- 

 lutely nothing as to their physical nature. It would be valuable to have 

 photographic records of them, especially of the nucleus and corona, which 

 undergo changes from day to day; and hence such a means of recording 

 these changes as photography ofl^ers would be the best, beyond comparison, if 

 the light of the comet were sufficiently intense to imprint itself. 



On the appearance of Donati's comet in 1858 I made several unsuccessful 

 attempts to delineate it with my reflector on a collodion film, but without 

 success ; and on the appearance of the comet of the present year I made 

 numerous attempts to depict it, not only with my telescope, but also with a 

 portrait-camera; but, even M'ith an exposure of 15 minutes (minutes, not 

 seconds), I failed in getting the slightest impression, even with a portrait- 

 lens. Hence the conclusion may be arrived at that the actinic ray does not 

 exist in sufficient intensity in such a comet as that of 1861 to imprint itself, 

 and therefore photography at present is inapplicable to the recording of the 

 appearances of these wonderful bodies. 



I now return to Heliography. Experiments conducted at the Kew 

 Observatory by my request have shown that, for an image of the sun of any 

 given size, when once the aperture of the telescope has been ascertained 

 which IS sufficient to produce the picture with the necessary degree of 

 rapidity, it is not beneficial to increase that aperture ; that is to say, no more 

 details are depicted, nor does the picture become sharper, so as to bear a 

 greater subsequent enlargement in copying, than when the smaller aperture 

 IS used. It has also been established, experimentally, that it is not well to 

 enlarge the image beyond a certain point by increasing the magnifyine 

 power of the secondary magnifier and thus to cause the rays to emerge at a 

 very great angle. These results are such as I should have anticipated • but 

 as It was, nevertheless, desirable to produce i)ictures of the sun's spots.'with 

 a view to their close study, on a scale considerably greater than the pictures 



