388 



SCIENCE. 



[N. S. Vol. VIII. No. 195. 



It was not until the study of the pecu- 

 liarities of comet tails with portrait lenses 

 that we knew anything of the strange phe- 

 nomena shown by them. It may be said 

 that onr knowledge of the extremely rapid 

 transformations in the tails of comets dates 

 from the photographs of Swift's comet of 



1892, taken at the Lick Observatory with 

 the lens previously mentioned and similar 

 ones taken of the same object by Professor 

 Pickering at Arequipa, Peru. Although 

 the great comet of 1882 was successfully 

 photographed, it showed no phenomena not 

 known and already seen with the telescope. 

 "While only an insignificant affair visually, 

 and but fairly visible to the naked eye. 

 Swift's comet showed upon the photographic 

 plates the most extraordinary and rapid 

 transformations yet seen in any comet. 

 One day its tail would be separated into at 

 least a dozen individual streams and the 

 next present only two broad streamers, 

 which a day later had again separated into 

 numerous strands, with a great mass, ap- 

 parently a secondary comet, appearing some 

 distance back of the head in the main tail, 

 with a system of tails of its own. This re- 

 markable appearance was the first known 

 of its kind, though it was repeated in the 

 photographs of Eordame's comet of 1893 

 taken by Professor Hussey. These peculiar 

 phenomena seem to be a production of the 

 comet itself — a result of the forces at work 

 in the head of the comet. 



The photographs of Brooks' comet of 



1893, also secured with the Willard lens, 

 showed such an extraordinary condition of 

 change and distortion in the tail as to sug- 

 gest some outside influence, such as the 

 probable collision of the tail with some re- 

 sisting medium, possibly a stream of me- 

 teors, such as we know exist in space. The 

 long series of photographs obtained of this 

 comet frequently showed great masses of 

 cometary matter drifting away into space, 

 probably to become meteor swarms. One 



of the pictures showed the tail of the comet 

 streaming irregularly as if beating against 

 a resisting medium and sharply bent at 

 right angles near the end, as if at that point 

 it encountered a stronger current of resist- 

 ance. All of these wonderful phenomena 

 would have been unknown to astronomers 

 had it not been for these photographs, and 

 the comet, instead of proving to be one of 

 the most remarkable on record, would have 

 passed without special notice. Though these 

 phenomena were so conspicuously shown, 

 scarcely any trace of the disturbance was 

 visible with the telescope. On account of 

 the apparent insignificance of the comet 

 visually, no photographs were made of it 

 elsewhere during its active period. 



In the matter of discovery the photo- 

 graphic plate has accomplished a very great 

 amount in certain directions. In spectro- 

 scopic work it has a field singularly suited 

 to display its possibilities. In this direc- 

 tion it deals not alone with what can be 

 seen, but it enters into the unseeable regions 

 where the eye takes no cognizance of things. 

 For though it is partly blind to the light 

 which affects the ej'e, it can readily pene- 

 trate regions where we in turn are blind. 

 And it is in this direction mainly where the 

 photographic discoveries in spectrum analy- 

 sis are immediately concerned, since it ex- 

 tends our vision into the invisible regions 

 of the spectrum. The result must necessa- 

 rily be one of discovery. It not only faith- 

 fully records spectral lines that cannot other- 

 wise be seen, but by special treatment of the 

 plate it also registers those visible to the eye 

 and permits their accurate measurement. 



From Doppler's principle it is known 

 that the spectral lines have a normal posi- 

 tion only while the object whose light is 

 under examination is motionless in the line 

 of sight. When it is in motion to or from 

 us these lines are displaced from their 

 normal position, in the first case towards 

 the violet region of the spectrum, and in 



