i9o8.] OF DANIEL'S COMET. 13 



September 12. — I could not see it with the naked eye though 

 I tried hard. Not bright in guiding telescope. Sky not pure. 

 Strong dawn. 



The photographs taken here of this comet were made with the 

 lo-inch, the 6.2-inch and the 3.4-inch portrait lenses of the Bruce 

 telescope of the Yerkes Observatory. The plates used were Seed 

 2y Gilt Edge. They were backed with a dark red paste made of 

 burnt sienna and caramels. 



Much trouble was experienced from cloudy weather and bad 

 skies. Every opportunity was taken advantage of, however, to 

 secure photographs of the comet. I am greatly obliged to my 

 friend Dr. S. A. Mitchell, who guided for me on several mornings 

 that work with the large telescope would otherwise have prevented 

 photographs being secured. On a few mornings Dr. [Mitchell at- 

 tached his small camera with a Goerz double anistigmat lens of i^- 

 incli aperture and 6-inch focus .'. a/f ^ /4.8> on to the Bruce mount- 

 ing, and secured some negatives which showed a greater length of 

 tail than was possible with the other lenses. 



Following is a list of the photographs made with the Bruce 

 telescope : 



In the column marked " Lenses," a is the lo-inch Brashear doub- 

 let, b the 6-inch, c the 3.4-inch and d the i^-inch Goerz lens. 



In conclusion it would seem that we have to deal with several 

 different kinds of physical phenomena in the study of comets. 

 These are doubtless closely related and are probably the same phe- 

 nomena acting under different conditions. 



There is the regular production of the tail through the repellent 

 action of the sun's light. The tail forming particles in this case 

 will be very small. They may go out from the comet as a broad 

 stream or they may produce several streams more or less narrow. 

 The direction of these various rays are dependent, to some extent, 

 on an exciting and directing force in the comet itself, but the general 

 direction will be more or less influenced by light pressure. These 

 streams, or rays, will be more or less uniformly straight or curved — 

 almost always straight or nearly so. They may be broken or 

 abruptly deflected but this will be due to some influence encountered 



