PHOTOGRAPHY OF THE LUNAR SURFACE. 107 



ciency of large objectives by causing those rays which have the strong- 

 est reducing action on the silver salts to be brought to a common 

 focus. By means of this improvement, photography, compared with 

 direct vision, found itself in possession of an advantage which might 

 have been theoretically predicted. 



For a given diameter of the objective, the chemical rays emitted by 

 a star will naturally produce an image richer in detail than that pro- 

 duced by the visible rays capable of affecting the retina. As is well 

 known, the image of a luminous point is a small circular spot, the 

 diameter of which is proportional to the wave length emitted. Actinic 

 rays, more refrangible than the visible rays of the spectrum, correspond 

 to a shorter wave length, and consequently give rise to a sharper and 

 more detailed image if these rays are made to converge by a suitable 

 choice of lenses. 



The second difficulty has been eliminated, at least in ordinary cases, 

 by the employment of an auxiliary telescope mounted coaxially with 

 the photographic telescope. It therefore becomes easy to observe a 

 star directly during the whole duration of the exposure, and to maintain 

 the image precisely at the intersection of two fine cross hairs, by adjust- 

 ing, as often as may be necessary, the mechanism controlling the motion 

 of the telescope. In this manner the relative immobility of the stars 

 with respect to the plates may be assured. This method can, how- 

 ever, only give its best results in the hands of a watchful and skillful 

 observer. It enables, by means of prolonged exposures, sharp images 

 of the smaller stars, suitable for the most precise measurements, to be 

 obtained. This advance, which is also due to MM. Paul and Prosper 

 Henry, has rendered possible the great international undertaking of 

 mapping the heavens. 



The method we possess of prolonging the exposure, almost without 

 limit, has in certain cases a great advantage. It gives rise to an indefi- 

 nite accumulation, so to speak, of luminous impressions at a given 

 point on the plate, and thus reveals perceptible images of objects too 

 dim to affect the retina. By this means vast regions of nebulous 

 matter and numerous small planets have been discovered which no 

 eye-piece, associated with the same objective, could have been able to 

 reveal. 



An amelioration of this difficulty has also been obtained in another 

 manner, for chemists have succeeded in increasing the sensitiveness 

 of photographic plates, especially by the substitution of gelatine for 

 collodium as a body for the silver salts. In certain cases the duration 

 of exposure has been reduced to a few hundredths or even thousandths 

 of a second, and thus photographs of objects in motion, too rapid to be 

 analyzed by the eye, have been obtained. In astronomy we may cite, 

 as examples, the photography of sun spots and of granulations of the 

 photosphere, as well as meteoric trajectories. For such a brief expo- 

 sure only an approximate coincidence between the movement of the 



