184 Mr. J. N. Lockyer. 



through 180. There is no statement in Dr. and Mrs. Huggins's 

 paper that this has been done. 



Finally, I may point out that with the above values, and assuming 

 that the nebula line exactly coincides, as Dr. Huggins says that it 

 does, with the least refrangible of the nitrogen double, the difference 

 in position between it and the magnesium fluting is less than a quarter 

 of the distance between the two D lines, and I have shown that this 

 difference may easily arise from instrumental errors. 



C. Observations by a New Method. 



The laboratory work having shown the numerous sources of error 

 connected with observations where great accuracy is attempted, it 

 seemed to me that it was quite hopeless to attempt very accurate 

 observations of nebulae in the ordinary way, where the conditions are 

 not nearly so favourable as in the laboratory. 



I have already pointed out that unless it can be demonstrated that 

 the collimator of the spectroscope is absolutely in the optic axis of 

 the telescope employed, one series of observations alone is worthless. 

 Again, the greater the dispersion employed, the greater generally 

 will be the weight of the spectroscope, and the less the stability of 

 the apparatus. Finally, as the telescope must necessarily be in 

 motion, the conditions are constantly liable to change by the varying 

 dispositions of the various parts of the apparatus. 



It struck me that these difficulties could be to a great extent over- 

 come by the use of a siderostat, in which case a spectroscope of any 

 weight could be employed, as it was no longer necessary that it should 

 be in. motion. To test this method, arrangements were made for 

 observing the spectrum of the nebula in Orion. A 12-inch side- 

 rostat and the 10-inch object-glass of the Science Schools equatorial 

 were employed, in conjunction with an optically perfect Steinheil 

 spectroscope belonging to the Physical Laboratory, and placed at my 

 disposal by Professor Biicker. 



The observations commenced on November 27th. 



The following account is based upon the records in the note book, 

 further explanatory additions having been made where necessary. 



November 2*7th. The 10-inch object-glass from the equatorial was 

 supported in a semi-circular wood block, on an adjustable lantern 

 tripod, which was sunk about 6 inches in the ground and the top 

 perfectly levelled. By carefully sighting a lamp supported at the 

 siderostat, the collimator of the spectroscope was placed in a line 

 with it. The object-glass was then put in proper line, and adjusted 

 by observations of Aldebaran with a reflecting eyepiece, which was 

 supported in front of the slit, and so arranged that when an object 

 was in the centre of the field it was also on the slit. Aldebarau was 



