Dreyek — A Survey of the Spiral Nebula Messier 33. 5 



nebula, still it is obvious that photography^ possesses great advantages 

 over direct micrometer measures. A photographic plate covers a 

 larger area than the micrometer screw can range over, and therefore 

 ofc's a larger selection of comparison stars, while it at the same time 

 enables the observer to measure these without taking transits, and 

 thereby introducing systematic errors. JsTobody has yet succeeded in 

 finding a nebula having proper motion or annual parallax ; and yet it 

 would be of the greatest value for our conception of the universe if 

 discoveries of this kind could be made, on account of the very peculiar 

 distribution of the nebulse over the heavens, the great majority of 

 them being massed nearly at the poles of the Milky AYay, while one 

 peculiar class — the nebulte having gaseous spectra — nearly all lie in 

 or close to the Milky AYay. 



For these reasons, I inquired some years ago from Dr. Isaac 

 Eoberts whether he would place some of his excellent photographs 

 of nebuloe at my disposal, for the purpose of measuring them. 

 Dr. Eoberts most kindly consented to do so; and having obtained from 

 the Government Grant Committee a grant for the purpose, I had a 

 measuring machine constructed by Messrs. Troughton and Simms 

 At Dr. Eoberts' request, I commenced operations (after having made 

 myself familiar with the instrument by measuring some plates of the 

 Pleiades) by examining two plates of the spiral nebula M. 33, the 

 results of which work I shall now describe. 



The instrument is designed for measuring either position angles 

 and distances, or rectangular coordinates, one at a time. I have 

 hitherto only used it for the latter purpose.^ The plate-holder is 

 arranged to hold plates of 16 cm. square (the international size), or 

 10 cm. square (Dr. Eoberts'), it is placed in the centre of a rotating 

 position circle of 25-5 cm. diameter, graduated to 10', and read by 

 two verniers, and supplied with a clamp and slow motion. The 

 square frame of the holder and circle can slide up and down a slide 

 (which I call the B-slide), inclined about 45° to the horizontal plane, 

 and is counterbalanced by a weight held by a chain passing over a 

 pulley. It is provided with a slow motion and clamp, which it has 

 rarely been found necessary to use. On the left-hand side the frame 

 is supported on two small wheels running on a plane surface ; on the 

 right-hand side there are two rectangular Y's resting and sliding on a 

 carefully turned steel cylinder, the perpendicularity of which to the 



^ The measuring machine in use at Green-svich is of a somewhat similar con- 

 struction. See Monthly Notices, 11. A. S., vol. Iviii., p. 327. 



