368 ANNUAL EEPOET SMITHSONIAN INSTITUTION, 1910. 



extension of the polar axis. Further, in order to lighten the great 

 weight of the moving portions, it rests at its lower part in a bath of 

 mercury; a large cylinder in the form of a mill wheel, and which 

 may be seen there, plays the part of a float. In all details the de- 

 signers have profited by the experience acquired elsewhere. 



In order to take advantage in every possible way of the light at 

 their disposal various combinations of auxiliary mirrors may be 

 used to give an instrument of various equivalent focal lengths. For 

 instance, the telescope, with its great parabolic mirror alone, has a 

 focal length of 7.6 meters and an angular aperture of 1 to 5. In 

 this form it is adapted for very faint nebulae. In conjunction with 

 a small hyperbolic mirror placed near the principal focus it forms 

 an instrument similar to the Crossley reflector of the Lick Observa- 

 tory. It has a focal length of 30.60 meters, and at the same time, 

 and this is very remarkable, the images are very greatly improved 

 from certain aberrations. Other arrangements allow foci of 21.40 

 and, indeed, of 45.50 meters. In the last case, especially useful in 

 the study of the brighter stars, the light beam is sent down through 

 the iDolar axis — which is hollow — and from there it passes into a 

 subterranean chamber under the pier, where it may be analyzed by 

 means of a fixed spectrograph, free from variations of temperature. 

 We ought to say that in the mind of Ritchey even a focal length of 45 

 meters is not enough for his work on the moon. He proposes by more 

 or less complicated reflections of the beam of light to photograph our 

 satellite at a focal distance of 150 meters ! 



Eitchey's method of work deserves description. As at the Lick 

 Observatory, the photographic plate carrier is provided with two 

 micrometer screws at right angles to each other for producing the 

 motions of the plate necessary for "following." It may also be 

 turned in its own plane. By means of two different eyepieces at- 

 tached to the plate carrier two stars may be seen; the first, which 

 is kept constantly in view, is the guiding star, properly speaking; 

 the second serves to correct the differential effects of refraction which 

 would result in a rotation of the field. From time to time it is 

 noted whether the star in the latter has left its cross wire, and a 

 slight rotation of the plate holder suffices to bring it back to its 

 place. Nor is that all ; the focal length of the telescope may change 

 during the course of an exposure lasting some 8 or 12 hours and 

 continued during several nights. This is corrected every half hour 

 by the knife-blade method of Foucault, susceptible of the precision 

 of one- fortieth of a millimeter. For accomplishing this it is, of 

 course, necessary to remove the plate, but a system of stops allows 

 him to put it back into its proper position again. 



Ritchey, helped by an incomparable sky, has obtained plates which 

 prove that such refinements are not illusory. His nebulae, of the 



