,907.] PERTAINING TO WORK WITH A PORTRAIT LENS. 427 



30, all this definlteness of form had disappeared and the tail was 

 very wide and dififused. 



The lower plate (exposure i hour) is very interesting, but the 

 main features of the tail are lost in the reproduction. Both photo- 

 graphs have suffered greatly in making the half-tones. 



The Lunar Surface under Various Kinds of Light. 



Plate VI shows two photographs of the moon. The size of the 

 lunar image on photographs with a portrait lens (a half inch in 

 diameter with the lo-inch telescope) is too small to be of any impor- 

 tance in the study of its crater and mountain-scarred surface. Such 

 photographs, ordinarily, are, therefore, not worth the making. But 

 there are conditions under w^hich the moon may be photographed to 

 advantage with these lenses — nor for a study of the craters and 

 mountains, however. The first of these pictures (enlarged) shows 

 the new moon with the slender sunlit crescent embracing the dark or 

 night part, v/here no direct sunlight reaches the surface, or in other 

 words it is the " old moon in the new moon's arms," which some- 

 times forms such a beautiful picture in the western sky at the van- 

 ishing of twilight when the moon is but a few days old. With the 

 exception of the bright crescent, what we see is the lunar night, but 

 it is a full " moon " night, for the illumination is entirely by sun- 

 light reflected from the surface of the earth onto the night side of 

 the moon. At that time if one were placed on this night part he 

 would have seen the earth shining in the night sky like a great round 

 moon (nearly full) some thirteen times bigger than the moon ever 

 appears to us. The distinctness with which the lunar surface is 

 shown in the photograph (with only 20 seconds' exposure) gives 

 an idea of how brilliant the full earth must be when shining in the 

 lunar night. This picture was made for comparison with the full 

 moon and w4th the totally eclipsed moon, for the surface is then 

 shown under three different kinds of illumination, i. e., direct sun- 

 light (full moon) reflected sunlight (earth lit moon) and refracted 

 sunlight (totally eclipsed moon) to see if any difference could be 

 detected in the appearance of the surface as affected by these various 

 illuminations. Portrait lenses are specially suited for this purpose. 



The second picture of this plate is a photograph of the totally 



