202 STELLAR PHOTOGRAPHY. 



Trails. 



Various advantages accrue to the method of photographing the stars without 

 moving the telescope. Each star as it passes through the field leaves a trail which 

 appears on the plate as a fine line, forming part of a circle having the pole as a 

 centre. The first advantage of such a line is, that it is distinguished with certainty 

 from a defect in the plate. When the photograph is to be used as a measure of 

 stellar magnitude, the trail shows that the plate has the same sensitiveness through- 

 out. In ordinary plates this condition appears to be perfectly' fulfilled. The trails 

 appear as lines whose intensity is perfectly uniform within the limits of accuracy 

 of which the comparison is capable. Small differences in light are much more 

 perceptible in the trails than in the circular images formed when the telescope is 

 driven by clockwork. The principal objections to photography as a means of 

 determining the brightness of the stars are, first, that for slight differences in 

 brightness the photographic images differ less than the real images. This objec- 

 tion is, however, counterbalanced by the possibility of repeating indefinitely doubtful 

 measures, and of comparing a large number of similar trails under nearly the same 

 conditions. Secondly, a variation in focus in different portions of the plate may 

 affect the measures. A star out of focus may leave a broad trail and appear 

 brighter than another which gives a narrow trail in consequence of its being more 

 neai-ly in focus. The fact that the photographic intensity will vary greatly with 

 the color can scarcely be called an objection. We wish to know the true relative 

 intensities of the light of the stars, and not merely their relative bx'ightness as 

 judged by the eye. As long as the spectra of the objects compared are the same, 

 that is, as long as the light of any given wave-length emitted by each bears the 

 same proportion to the whole, all methods of measurement will give the same 

 result. In other words, the relative intensity will appear to be the same, whether 

 it is measured by the eye or by the sensitive plate. This is the more precise state- 

 ment of the case which is commonly expressed by saying that the color is the 

 same. When the spectra differ, and the colors are unlike, no single number will 

 properly express the ratio of the two lights. The onlj' true comparison is by a 

 series of numbers which express the ratio of the light for each different wave- 

 length. When, therefore, we say that a red and a blue star appear equally bright, 

 we merely indicate that the entire radiation affects the eye equally. The visual 

 result will not in general differ much from what would be attained if all the light 

 had a wave-length .00006 cm., or 6000 ten-millionths of a millimetre. The photo- 



