October 26, 1900.] 



SCIENCE. 



627 



one would expect to be strong, do not 

 so appear ; and vice versa. Images which 

 would be strong for the right and left 

 edges separately need not be so when the 

 former are superimposed on the latter. 



With these remarks I believe to have 

 given a sufficient account of these interesting 

 diffractions. I began the work since in all 

 my reading in physics I had never seen a 

 reference to these ubiquitous phenomena, 

 and I hoped with the present paper to fur- 

 nish at least one contribution of known 

 whereabouts. In the course of the work I 

 found much greater subtlety than I was 

 prepared for, and some of the cases given 

 are available for more rigorous treatment 

 elsewhere. 



Cakl Barus. 

 Beown University, 



Providence, E. I. 



THE CB088LEY BEFLEOTOB OF THE LICK 

 OBSERVATORY. 



The leading article in the June number 

 of Tlie Astrophysical Journal has the above 

 title and was written by Professor Keeler. 

 It is a very full account of the instrument 

 and of the work accomplished with this 

 telescope since its installation on Mt. 

 Hamilton. I am very glad to comply with 

 the request of the editor to furnish an ab- 

 stract for Science. 



The frontispiece of the number is an ex- 

 cellent heliogravure plate of the ' Trifid ' 

 nebula in Sagittarius, from a negative 

 made with the Crossley reflector. In this 

 connection it ought to be said that no 

 known method of reproduction gives all 

 the detail to be seen on the original nega- 

 tives of such subjects. There are also half- 

 tone illustrations, from photographs, of the 

 details of the telescope and its observatory. 



This telescope was made by Dr. A. A. 

 Common, of London, in 1879, and used by 

 him until 1885, when he decided to build 

 one of 5 feet aperture. He then sold the 



3-foot instrument to Edward Crossley, Esq., 

 of Halifax, England. For the construction 

 of the instrument and for photographs ob- 

 tained with it. Dr. Common was awarded 

 the gold medal of the Eoyal Astronomical 

 Society in 1884. 



Mr. Crossley built a very complete ob- 

 servatory and dome for the telescope and 

 used it for a number of years. The climate 

 of Halifax was not adapted to the use of 

 reflectors, however, and in 1895, at the 

 request of Professor Hoi den, then director 

 of the Lick Observatory, Mr. Crossley pre- 

 sented the telescope and its dome to this 

 institution. The expenses incurred in 

 transporting it from England and in erect- 

 ing a suitable building on Mt. Hamilton 

 were borne by friends of the Lick Observ- 

 atorj% principally residents of California. 

 It was mounted here the same year. Its 

 dome is situated on a spur of the mountain 

 some 350 yards to the south of the main 

 observatory and about 150 feet lower. 

 The building contains, in addition to the 

 dome and vestibule, a photographic dark- 

 room, a study, a room for apparatus and 

 storage, and a room for the hydraulic ma- 

 chinery which was used in England to re- 

 volve the dome. The present site is such 

 that the hydraulic system which is used 

 for the large refractor is not available for the 

 Crossley reflector. The dome is turned by 

 hand bj' means of an endless rope and a 

 set of gears working in a cast-iron rack 

 bolted to the inside of the sole plate of the 

 dome. The dome is covered with sheet- 

 iron, the framework being of iron girders. 

 It is of the usual form, with a shutter in 

 two parts which are rolled to each side, ex- 

 posing a slit six feet wide. The slit extends 

 well beyond the zenith. From the inside 

 of the dome is swung a system of platforms 

 around the telescope for the observerto stand 

 upon. The cylindrical walls upon which 

 the dome rests are double, 36;^ feet inside 

 diameter. The dome itself is 38 feet 9 



