366 PROCEEDINGS OF THE AMERICAN ACADEMY 



to be erected at a great elevation the advantages of the present plan are 

 at once apparent. Many nights of observation would be secured which 

 otherwise would be lost owing to the wind and. cold. The simplicity 

 in the construction of the building would be a great advantage, as a 

 large dome in so exposed a situation would be kept free from snow 

 with much difficulty, and might be a source of danger in winter storms. 

 If found impracticable to observe during the winter, it would be pos- 

 sible to have a duplicate mounting below, and remove the lens and 

 mirror from one to the other. 



It is evident tliat the saving of cost would be very great not only in 

 the observatory building and dome, but in the tube, observing chair, 

 clockwork, etc. 



If a reflector could be constructed whose surface was the portion of 

 a paraboloid whose abscissa equalled that of the focus, the instrument 

 could be much simplified. No object-glass would then be required, the 

 reflector taking the place both of mirror and lens. All the light 

 intercepted by the objective would thus be saved, and but a single 

 surface need be adjusted and corrected. "With the advance in mechan- 

 ical methods this does not seem wholly impracticable, especially with a 

 mirror of long focus. Since the final correction must always be made 

 by hand, it makes but little difference what is the exact form of the 

 surface. 



In any case it would be a great advantage that the mirror could be 

 reground, repolished, or resilvered without moving it fi'om its place. 

 It would only be necessary to place it horizontally, and the grinding 

 machinery could be kept permanently near it. If plane, the perfection 

 of its form could also be tested at any time by setting it on edge, and 

 viewing the image it reflected by a collimating eyepiece attached to the 

 large telescope. Another method would be to place a heliotrope a few 

 hundred yards to the north or south of it, and the light from this would 

 form an excellent artificial star, available whenever the sun shone. 



The greatest advantage is the rapidity with which observations 

 could be made. No more time would be lost in identification than 

 with a transit instrument, so that a large number of objects could be 

 examined in the course of a single hour. Any one who has worked 

 with a large telescope knows how much time is lost in opening and 

 closing the dome and in finding and identifying minute objects. 



Let us now consider to what purposes a large telescope mounted as 

 suggested might be applied. 



I. Sweeping. For the discovery of new objects this mounting pre- 

 sents especial advantages. It might be used for the detection of new 



