i9o8.] OF DANIEL'S COMET. 5 



are common to the Lick and the Yerkes plates. Several of the 

 Yerkes photographs show very little on account of clouds and thick 

 sky on the dates in question. 



Another fine series of photographs of the comet was made by 

 ]\I. F. Quenisset of M. Flammarion's observatory at Juvisy, France. 

 The interval of some six hours makes this series specially valuable 

 for comparison with plates taken in this country. I am greatly 

 indebted to M. Quenisset for enlarged prints from eleven of these 

 pictures. Out of these, there are eight dates which were dupli- 

 cated at the Yerkes Observatory. A comparison of these photo- 

 graphs is of extremely great interest, and though there is but little 

 material from which to accurately determine the amount of motion, 

 progressive outward displacement, especially in the streamers, is 

 strongly shown. A study of these photographs clearly shows how 

 uncertain it is to connect the details of any two dates. Of course 

 a disturbance may extend over several days and the matter from 

 it still be visible, but any particular detail would not probably live 

 through from one date to another. In some of M. Quenisset's 

 photographs the change has been so great that it is almost impos- 

 sible to be sure of the same features six hours after. What is 

 quite evident, however, in the comparison, is that the structure of 

 the tail (the streamers) has a decided outward motion as a whole; 

 at the same time there is a diffusion effect that constantly tends to 

 destroy the details. 



Some of these comparisons follow : 



July ip, Juvisy Plate. — There is a principal narrow ray that 

 separates into two rays some distance out. A dark space intervenes 

 between it and a broad streamer south, whose north edge is very 

 definite. There is a very decided change shown in the Yerkes 

 photograph. Two new short rays have appeared on the south side. 

 The north ray has become broad and diffused and irregular. The 

 changes are so striking that one can hardly be sure of the same 

 features, though there is a general resemblance. 



August II. — In the Juvisy picture there are four distinct rays. 

 The two middle ones diverge from a point close to the head. These 

 two are clean cut in the Yerkes plate. The south one has become 



