December i, 1910] 



NATURE 



141 



To maintain a nearly constant temperature, Prof, 

 Ritchev now encloses the greater part of the telescope 

 during' the daytime in a light, removable room or 

 chamber, with' insulating walls, which he calls the 

 •canopv." The walls of this consist of four thick- 

 nesses of fine woollen blankets quilted between covers 

 of white canvas, while the floor is of mats two inches 

 thick, made of cheap woven hair, sewed between 

 covers of heavv canvas. At the upper south portion 

 of the canopy' the head end of the skeleton tube 

 projects, and this opening is closed airtight by a 

 folding wooden cover lined with wool felt. In addi- 

 tion to these precautions the large mirror is protected 

 bv a short cast-iron tube, and by the airtight covers 

 ^vhich protect its surface. Arrangements are made 

 for moving the canopy easily and entirely out of the 

 wav of the telescope when in use, and replacing it 

 when observing is completed. While the telescope 

 was protected in the above manner, a sun shield was 

 used to reduce the daily variation of the dome. This 

 consists of gores of heavy white canvas laced to a 



Fig. 2. — The new photographic plate carrier on the 6o-inch reflector. 



strong framework of steel pipe. T he canvas was 

 thus retained two feet from the steel covering ^of the 

 dome, due provision being made for the free "circula- 

 tion of the air beneath the canvas. In this way*the 

 daily variation in the dome was decreased to jo^. F. 

 in July, while the change in focus of the mirror was 

 reduced to o"02 inch. 



By the combination of shield and canopy, the in- 

 side daily variation of temperature in the latter was 

 only 3'8° F. in August and September, and the 

 apparent alteration of focus reduced to 0*005 inch. 

 Prof. Ritchey proposes, in future, two improvements 

 when he still further hopes to reduce this daily ampli- 

 tude of variation, first to place in the canopy a small 

 refrigerating apparatus with a controlling thermostat, 

 and, second, to enclose the complete telescope in the 

 canopy. 



With these refinements in controlling temperature 

 changes, he adopts the knife-edge method of focussing 

 the stellar imatjes, a most important consideration in 

 stellar or nebular photography. By this means he is 

 able to locate the focal plane to within 0001 inch. 

 AA'ith the help of his new olate-carrier, the focal plane 



XO. 2144, VOL. 85] 



and the plane of the film of the photographic plate 

 can with certainty be made to agree within 00003 inch 

 While making an exposure he has occasionall}- to re- 

 move the plate to check the position of the focal plane 

 of the mirror. Since the adoption of the canopy and 

 shield he has found that re-focussing about ever)' half- 

 hour in the early part of the night, and about every 

 three-quarters of an hour after 11 p.m., is sufficient 

 for accurate working. 



The efficiency of the whole instrument is such that 

 Prof. Ritchey states : — 



" .All of the uncertainties which usually occur in making 

 long exposures with very large instruments are eliminated. 

 A plate can be exposed night after night, if desired, with 

 the assurance that no error in focus greater than one or 

 two-thousandths of an inch can occur, and that no rotation 

 of field can take place without immediately being detected 

 and corrected. Both of these conditions are absolutely 

 necessary for the finest results with an instrument so 



powerful and sensitive as the 60-inch On the best 



of these negatives, with exposures of eleven hours, the 

 smallest star-images are 1-03 seconds in diameter." 



To ensure the finest of final products. Prof. Ritchey 

 lastly abandons the use of rapid plates, which, as is 

 well known, are always associated with coarseness of 

 grain, and employs Seed "23" plates almost exclu- 

 sively. • ■ 



A close examination of the reproductions of some 

 of the nebulae which he publishes with his latest com- 

 munication indicates in a striking manner the wonder- 

 ful sharpness and richness in detail of his photographs. 

 It is interesting in this respect to compare Ritchey 's 

 photograph of the spiral nebula Messier 51 Canum 

 V'enaticorum, with that of Keeler, reproduced in 

 vol. viii. of the •" Publications, of the. Lick Observa- 

 tory " (plate 47), those of Isaac Roberts, in vojs. i. and 

 ii. of his " Photographs of Stars, Star Clusters, and 

 Nebulae " (plates 30 and 15 respectively), and, lastly, 

 that by Ritchey himself, taken with the 2-foot reflector 

 of the Yerkes Observatory, and published in vol. ii. 

 of the ■■ Publication of the Yerkes Observatory " 

 (plate 29). 



Bearing in mind the differences in quality of the 

 reproductions to which references above are given, the; 

 superiority of Ritchey 's latest achievement is well 

 marked. 



In a more recent announcement (Monthl\ Notices, 

 R.A.S., vol. Ixx., Supplementary Number, No. 9), and' 

 dated September 17, Prof. Ritchey directs attention to 

 verv important conclusions which he is able to arrive 

 at -from his recent photographs. These are that the' 

 spiral nebulae are not only distinguished by ^many^ 

 sharply-marked characteristics from all other classes 

 of nebulae, but that the spirals themselves exhibit 

 marked differences from each other in regard to the 

 distribution of the nebulous stars, differences which, 

 as he states, possibly correspond to successive stages 

 of development. 



It is in the presence of such photographs as these,_ 

 and more especially those where the nebulae are of a, 

 spiral nature, that one's attention is directed to the 

 question of the origin of stars themselves. 



" All self-luminous bodies," as Sir Norman Lockyer 

 states in the first of his General Conclusions at the end 

 of his work 'The Meteoritic Hypothesis,' "in the 

 celestial spaces are composed either of swarms of 

 meteorites or of masses of meteoritic vapour produced by 

 heat. The heat is brought about by the condensation of 

 meteor swarms due to gravity, the vapour being finally 

 condensed into a solid globe." 



Such a photograph as that of Messier 51 seems to 

 represent the above words in picture form. Prof. 

 Ritchev, in commenting on these spirals, which he 

 has most recently photographed, says, that they all 

 " contain great numbers of soft star-like condensations 

 which I shall call nebulous stars. They are possibly stars 



