108 PHOTOGRAPHY OF THE LUNAR SURFACE. 



telescope and the apparent displacement of the star is necessary, and, 

 moreover, atmospheric undulations also cease to exert an injurious 

 influence on the image formed. 



III. — CHARACTERISTIC DIFFICULTIES IN THE REGISTRATION OF 



LUNAR IMAGES. 



All these advances have led to a kind of revolution in the methods 

 of observation. They have also led us to undertake the solution of a 

 great number of new problems in sidereal and physical astronomy, but 

 none of the methods we have been discussing give entirely satisfactory 

 results when applied to the moon. The employment of a finder makes 

 it possible to obtain regular disks for stellar images if the stars are not 

 too close to one another, but it does not eliminate the confusion of 

 adjacent details in an extended object which we seek to reproduce with 

 every possible refinement. It is necessary to adopt special mechanical 

 contrivances to follow the moon's motion, constantly variable both in 

 direction and in velocity. Various solutions to establish this concord- 

 ance have been proposed, and, on the whole, with success. At Paris 

 this has been accomplished in a very satisfactory manner by making 

 the telescope immovable. The plate holder being moved by a separate 

 motor, its orientation is therefore entirely under the control of the 

 observer. However, as the moon is not sufficiently luminous to pro- 

 duce a suitable image in a small fraction of a second, it has been impos- 

 sible up to the present to eliminate the effects due to the undulations of 

 atmospheric origin. These are almost completely beyond the control 

 of astronomy, and it is therefore necessary to select moments when they 

 make themselves least felt. Very often they render the theoretical 

 superiority of large objectives illusory. To show how harmful their 

 influence is, it is sufficient to say that at Paris after four years, during 

 which every opportunity which has. appeared to be favorable for photo- 

 graphing the moon has been utilized, only a dozen evenings, at most, 

 have given really satisfactory negatives which would stand strong 

 magnification. 



IV. — ADVANTAGES OF PHOTOGRAPHY OVER DIRECT OBSERVATION. 



It seems that in this respect direct observation must claim a distinct 

 advantage over photography. A very short time, indeed only a few 

 hundredths of a second, are necessary to affect the retina. 



The observer, with his eye at the ocular, can thus, if he is watching 

 the object which he wishes to examine, take advantage of the rare 

 intervals of calmness which are lost in photography. The nights 

 suitable for observing will then, in the former case, be much more 

 numerous. Notwithstanding this, however, there still remains a great 

 superiority for photography, for a single negative obtained under 

 favorable conditions includes an abundance of data and exact details 

 which it would be impossible for the most patient and skillful observer 



