250 SCIENCE PROGRESS 



find the image of the red star much fainter on substituting an 

 ordinary plate. If our photographic magnitudes are to mean 

 anything, we must keep to the same kind of plate. Strict uni- 

 formity in this respect has not been possible : the sensitising of 

 films is an art rather than a science and when a firm of plate 

 makers changes its artist, the plates undoubtedly change in 

 character. But it is hoped that the variations in character of 

 plate throughout the work have not been serious enough to 

 introduce large errors, though this is a point on which our infor- 

 mation is not very complete. Moreover this is not the only 

 trouble. Two stars showing similar photographic images on the 

 same plate may be made to give dissimilar images by slightly 

 turning the telescope so that the images fall on different points 

 of the plate : or by refocussing the telescope. The apparent 

 photographic magnitude may thus depend upon the distance of 

 the star from the plate centre and upon the particular focus 

 selected for the plate. The difference is so slight that it might 

 escape detection by the direct process of comparing images : 

 but it can be made unmistakably manifest in a very simple way. 

 It has been already mentioned that the plates of the Star Map 

 are ruled with a series of cross lines, called a reseau, dividing 

 up the plate into equal squares. Let us count the number of 

 star images in each of these squares for a large number of plates 

 and add together all the counts for each particular square : then 

 if stars photograph equally well all over the plate, the total 

 numbers for each particular square ought to tend to equality : 

 the stars are scattered sufficiently at random for this purpose. 

 Now it is found that on the plates of the Star Map the numbers 

 do not tend to equality : at the edges of the plate, the totals per 

 square are considerably less than nearer the centre — the in- 

 equality may be as great as i to 2. The increase, however, is 

 not maintained up to the centre : it reaches a maximum and then 

 falls off' again, unless the plate happen to be focussed in such a 

 position that the centre is most favoured. The phenomenon 

 depends, in fact, on the focussing of the plate : if it be focussed 

 for the centre, then the total per square will be greatest at the 

 centre and will fall off steadily towards the edges; but it is 

 customary to push the plate a little further in than this, so that 

 the region of best focus is a ring intermediate between the centre 

 and the edges and on this ring the total-per-square is greatest, 

 falling off" both towards the centre and towards the edges. 



