l£ OBSERVATIONS OF A COJIET. 



4th and for the 19th, the result of Which is, that On the 4tf> 

 the illumination was 119° 45' 9", as represented in PI. I, 

 fig. 3, and that on the 19th it had gradually increased to 

 124° 22' 40", of which a representation is given in tig. A* 

 Both phases appear to me sufficiently defalcated, to prove 

 that the comef did not shine by light reflected from the sun 

 only ; for had this been the case, the deficiency I think 

 would have been perceived, notwithstanding the smallness 

 of the object. Those who are acquainted with my experi- 

 ments on small silver globules* wi'l easily admit, that the 

 same telescope, which could show the spherical form of 

 balls, that subtended only a few tenths of a second in dia- 

 meter, would surely not have represented a cometary disk as 

 circular, if it had been as deficient as are the figures which 

 give the calculated appearances. 



indicates it to jf these remarks are well founded, we are authorised to 

 be luminous of 1,11,10, • „ . ,„ 



iuelf. conclude, that the body ot the comet on its surface is selr- 



luminous, from whatever cause this quality may be derived. 



The vivac'ity of the light of the comet also had a much greater 



resemblance to the radiance of the stars, than to the mild 



reflection of the sun's beams from the moon, which is an 



additional support of our former inference. 



Which is far- The changes in the brightness of the small stars, when 



ther proved by ^ e are succe s&ivelv immerged in the tail or coma of the 



the stars seen J J ° • i , 1 , 



through the comet, or cleared from them, prove evidently, that they are 



tai! - sufficiently dense to obstruct the free passage of star-light. 



Indeed if the tail or coma were composed of particles that 

 reflect the light of the sun, to make them visible we ought 

 rather to expe< ,, that the number of solid reflecting parti- 

 cles, required for this purpose, would entirely prevent our 

 seeing any stars through them. But the brightness of the 

 head, coma, and tail alone, will sufficiently account for the 

 observed changes, if we admit, that they shine not by reflec- 

 tion, but by their own radiance ; for a faint object projected 

 on a bright ground, or seen through it, will certainly appear 

 somewhat fainter, although its rays should meet with no ob- 

 struction in coining to the eye. N<;w, as in this case we are 

 sure of the bright interposition of the parts of the comet, 



* See Phil. Traus. for 1805, p. 38, the 5th experiment. 



but 



