OOTOBEE 26, 1900.] 



SCIENCE. 



629 



The image of a star is kept bisected by 

 turning the proper screws attached to the 

 movable frame. 



Upon taking charge of the Lick Obser- 

 vator}^ in June, 1898, Professor Keeler de- 

 cided to devote his own observing time to 

 the Crossley reflector, notwithstanding 

 that his previous experience had all been 

 with refractors. Upon making a careful 

 examination of the instrument, he found 

 that a number of changes would be necessary 

 before satisfactory results could be expected. 

 Some of these were required on account of 

 the change in latitude and the different 

 climatic conditions existing at Mt. Hamil- 

 ton. The brick pier upon which the tele- 

 scope rested was found to be too high for 

 the greatest convenience and usefulness and 

 was lowered two feet. The polar axis was 

 found to work hard, the plan of mercury 

 flotation not being successful and the con- 

 struction being such that the friction was in- 

 creased in this lower latitude. This caused 

 the driving clock to run irregularly, and a 

 more powerful one was built at the observa- 

 tory from designs by Professor Hussey. A 

 further cause of irregularity was found in 

 one of the wheels of the differential gearing 

 for giving slow motion in right ascension. 

 As long exposure photographs near the 

 pole required a considerable degree of ac- 

 curacy in the position of the polar axis, 

 some time was spent in devising methods 

 for adjusting a telescope of this design. 

 The methods used for a telescope of the 

 ordinary construction do not suffice. One 

 very promising plan was to secure trails of 

 the stars near the pole on the same plate in 

 two positions of the telescope 180° apart. 

 Consistent results were not obtained, how- 

 ever, owing to the instability of the large 

 mirror. The axis was finally adjusted by 

 using a long finder for observations of 

 Polaris in the usual waj'^, a watch tele- 

 scope being fastened to the mounting in 

 such a way that an object on the southern 



horizon could be observed during the proc- 

 ess of shifting the iron pier. 



The resolving power of the telescope 

 was tested by visual observations of close 

 double stars, with the result that stars of 

 about the 8th magnitude and of nearly 

 equal brightness could be separated with a 

 magnifying power of 620, if as much as 0".3 

 apart. Stars of 5th magnitude and this 

 distance could not be seen double owing to 

 the increased amount of light. In con- 

 nection with these observations Professor 

 Keeler remarks : ' ' Although the theoretical 

 limit of resolution for a three-foot aperture 

 is not reached in these observations, I do 

 not think the mirror can do any better." 



It is, however, in photographic work that 

 the greatest field for the Crossley reflector 

 appears to lie, and it is largely with respect 

 to this line of work that any changes have 

 been considered. 



The ratio of aperture to focal length is so 

 large in the instrument (a little greater 

 than 1 to 6) that the field of view over 

 which the star images are sufficiently free 

 from distortion is only about 16', or one 

 inch, in diameter. The photographic equip- 

 ment was designed to use plates 3-^x4^ 

 inches in size. These are sufficiently large, 

 for even with this size the star images 

 show decided wings near the edges of the 

 plates. 



Several minor changes and improvements 

 were made in the ej'C-end apparatus. Metal 

 plate-holders were substituted for the wood- 

 en ones, as the latter could not be de- 

 pended upon to keep their positions through- 

 out the long exposures. Clamping screws 

 were added to hold the plate-holder firmly 

 in place. Spider threads were substituted 

 for the coarse wires in the guiding eye- 

 piece, and a system of mirrors added to 

 illuminate the declination thread. A small 

 electric lamp is used to illuminate the 

 wires, current being supplied from the 

 storage battery at the main observatory. 



