THE GREAT STAR MAP 251 



Photographers accustomed to ordinary cameras will read these 

 words with some surprise for they may not have noticed in their 

 experience any corresponding phenomenon ; but the reason of 

 this is simply that they use a different kind of lens — a " doublet " 

 made up of two lenses separated by an interval, and with this 

 combination the trouble does not occur. In sketching the early 

 history of the Star Map it was pointed out that the selection of 

 a lens was one of the important decisions taken by the Conference 

 of 1887, ^nd that it was decided not to use a doublet lens — chiefly 

 because of the expense. But in other connections stars have 

 been photographed with doublet lenses and it has then been 

 found that the inequality of distribution of images disappears or 

 is very considerably reduced. With the lenses used for the Star 

 Map, however, the inequality is marked. By noticing where the 

 total-per-square is a maximum we can ascertain to a nicety how 

 the plate has been focussed and whether the focussing has been 

 changed from one time to another. The position of the maximum 

 changes slightly with the season of the year, doubtless owing to 

 the expansion and contraction of the lens and the tube with 

 variations of temperature. We can even tell whether the plate 

 is tilted slightly to one side, for then the position of maximum 

 will be further from the centre on one side than on the 

 opposite. 



Hence it will be seen that the determination of the photo- 

 graphic magnitudes of the stars is beset with difficulties from 

 the outset. We must take into account the kind of plate 

 used and the position of the star on the plate if we are to 

 get comparable and accurate results. Nevertheless much 

 information can be obtained by very simple means if these 

 essentials are attended to. Suppose we take two plates from 

 the same batch and expose them for the same time on the 

 same night, one to a region in the Milky Way and the other 

 to a region far from it : and that we then count the total number 

 of stars upon each. There will be many more on the former 

 and we can find definitely what the ratio of the two numbers 

 is. Repeating the comparison with a longer exposure (still the 

 same for both regions), we shall get another ratio. From the 

 study of these different ratios for different lengths of exposure, 

 we get information bearing directly upon the two great 

 questions we have been studying — the existence of a solar 

 cluster and of fog in space : for we must ascertain whether 



