178 Mr. J. N. Lockyer. 



tant work since 1864 Dr. Huggins has employed only one or two 

 prisms as a rule, whilst now he states that he can use a dispersion 

 equal to nearly eight prisms of 60 in the case of the nebula in Orion, 

 and its use implies that this is the minimum dispersion that should 

 be used. I am rejoiced that this is so, if it be so ; and future obser- 

 vers, travelling over the ground of which I have attempted to make 

 a rough survey, will no doubt have better observations to work upon 

 than those on which I have depended. But although I am rejoiced 

 that increased dispersion is possible, I am so thoroughly acquainted 

 now with instrumental pitfalls that I cannot accept Dr. and Mrs. 

 Huggins's new value until we know more exactly how it has been ob- 

 tained, and until many observations, the conditions of which are 

 more fully described, have confirmed it. 



Dr. and Mrs. Huggins do not appear to have applied the same test 

 at the same time to the coincidence of the third nebula line with the 

 F line of hydrogen, so that whether the non-coincidence of the mag- 

 nesium was due to an instrumental error cannot be determined with 

 the facts before us. 



The observed difference between the nebula line and the magne- 

 sium fluting was nineteen of Dr. Huggins's present units, so that, 

 after all, if we only take his recent observations into account, we have 

 better evidence for the existence of magnesium in the nebulae than 

 we have for hydrogen in the white stars, so far as is evidenced by 

 the lines discovered by Dr. Huggins (see ante), for in their case the 

 coincidences do not occur within thirty units. 



I next refer to my own observations with high dispersion. 



B. Laboratory Observations with High Dispersion in connexion with the 

 Chief Nebula Line. 



Dr. Huggins's observations having led him to the conclusion that 

 the chief nebula line is coincident with the less refrangible member 

 of the double line of nitrogen near 500, and not with the magnesium 

 fluting, I first directed my attention to observations of these lines 

 and flutings in the laboratory, as the arrangements for observatory 

 work with high dispersion were not completed. 



The laboratory work was begun last May, and some of the photo- 

 graphic results were exhibited at the Royal Society Soiree in the 

 same month. It was, however, interrupted till the end of July, but 

 since the recess it has been taken in hand again. Dispersions vary- 

 ing from that given by a Liveing direct-vision spectroscope to that of 

 a Rowland grating of 12 feet 10 inches radius and 9'6 square inches 

 surface, with an eyepiece of 1'4 inches equivalent focus, have been 

 employed, in addition to which a Steinheil spectroscope with three or 



