208 Dr. and Mrs. Huggins. On the Principal 



unable, under any of the conditions of observing, to detect even a 

 suspicion of any softening of the more refrangible edge of the line, 

 much less the faintest indication of a " flare," and certainly not the 

 distinctive peculiarity of a " fluting." 



In the case of observations with small dispersion, the eye is helped 

 by placing the second line, which then appears near the first, behind 

 a bar fixed in the eye-piece. 



Observations of the nebula in Orion by eye, as well as the photo- 

 graphs of Mr. Common and of Mr. Roberts, show numerous small 

 irregularities in the brightness of the nebula, which give rise to a 

 closely-mottled appearance. As the length of the slit takes in a con- 

 siderable angular extent of nebula, several of these irregularities of 

 brightness or " mottlings " are usually included within it, giving to 

 the nebular lines an irregularly bright or blotchy appearance. As the 

 nebula is allowed to pass over the slit this blotchy appearance is seen 

 to vary in the size and in the number of the brighter patches, and also in 

 their brightness relatively to the less luminous spaces between them. 

 At the first glance, in some positions of the slit upon the nebula, the 

 lines, and especially the principal line as the brightest, appear almost 

 as if serrated at the edges. A little attention soon shows that this is a 

 purely physiological effect due to the greater brightness of the patches, 

 and that the brighter parts of the line do not really project beyond the 

 less brilliant intervals between them. One marked character of this 

 phenomenon is that both edges of the lines appear equally serrated, 

 and that there is no indication of a spreading of the brighter patches 

 towards the blue only. It is easily ascertained that this more or less 

 patchy condition is not peculiar to the principal line, for precisely the 

 same patches can be detected in the other two lines, and the patches 

 can be seen to correspond in number and in position within the 

 lines. 



These observations, repeated on several nights, have left no doubt 

 in our minds that the principal line is certainly as sharp and as 

 bright on the side towards the blue as on the less refi^angible side. 



On February 9, Professor Liveing scrutinised the character of this 

 line. His words are : " Observed the nebular line with various 

 widths of slit. The line always appeared sharply defined on the 

 more refrangible side, whether the slit were wide or narrow. On 

 gradually closing the slit, the line fined down to a very fine line. 

 The same appearance as to sharpness on the more refrangible side 

 was observable with a spectroscope of less dispersive power and with 

 eye-pieces of low power as with the higher dispersion and greater 

 magnification." 



The observations recorded in this paper appear to us to show con- 

 clusively : 



