ASTRONOMICAL TELESCOPES — PEASE 211 



find him at Avork with the spcctroLa-apli or some other instrument 

 which did not require the best seeing. 



In Figure 3 is shoAvn an alternative design for a 25-foot reflector 

 which coukl be used at all the various foci now employed in the 60 

 and the 100 inch reflectors. The mounting is essentially the same 

 as in the previous design, the principal changes being in the cages 

 and in the dome. The cage sliown hanging on the crane is tlie focal- 

 plane cage. In it the observer Avould sit, carried along in the tele- 

 scope, exposing his plates directly in the focus of the large mirror 

 without loss of light by a second reflection, thereliy gaining every 

 possible bit of light for the faintest nebulae and stars. Below the 

 mirror is shown an observer Avith the instruments best adapted for 

 use at the Cassegrain focus. At either end of the telescope pier there 

 would be a Avell connected Avith a constant-temperature room below, 

 which would permit the Coudc focus to be utilized. The rapid 

 change of cages Avould require a suitable oAerhead crane. Were 

 we to use such a crane as Ave noAv have in the curved dome of the 

 100-inch Hooker telescope, its Aveight Avould have to be counter- 

 balanced and its motion A-ery slow. In a cylindrical dome as shown 

 the crane has a straight travel and a direct hoist, and fast speeds 

 could be used. With a pyrex mirror, easily made to-day, and an 

 invar-steel tube, the images Avould not only be smaller on account 

 of the larger aperture but they Avould remain so because the curvature 

 of the mirror Avould change so little with temperature. 



Thus far our great reflectors have been used for observations of 

 planets, stars, and nebulse, but not of the sun. The domes are double 

 Availed to keep out the heat of day; the mirrors are inclosed in insu- 

 lated Avails to prevent more than a degree of change in their tem- 

 perature. Should Ave succeed, hoAvever, in making a mirror of fused 

 quartz, Ave could use the telescope for solar Avork as Avell. A giant 

 telescope Avould then yield a double return, because Ave could utilize 

 its poAver both day and night. 



