OF ARTS AND SCIENCES. 235 



Dr. Draper tried the effect of replacing the small convex mirror with 

 a fiat of IG inches diameter for photograpliic work, but the result was 

 not satisfactor}'. 



In the use of the reflector for photographic and spectroscoj)ic pur- 

 poses it was found extremely difficult, in fact impossible, to hold the 

 great mii-ror with sufficient firmness to keep the star image accurately 

 in place, without at the same time distorting the glass and injuring the 

 definition. In this respect the refractors had greatly the advantage, 

 though of course they were much inferior in the amount of light. 



Tlie 12-inch refractor needs no special description. It had a focal 

 length of 183 inches, and its color-correction was adjusted for the use 

 of aL-hromatic eyepieces, instead of the usual Huyghenian eyepieces. 

 The correction, however, did not vary materially from that of other 

 telescopes by the same makers, the difference of focal length between 

 the mean rays of the spectrum and H amounting to about j\ of an 

 inch. It was an excellent instrument for all visual purposes, and is 

 now owned by the Lick Observatory. For photographic purposes, 

 however, it was decidedly inferior to the 11-inch instrument which 

 succeeded it. 



The focal length of the 11-inoh telescope without its photographic 

 corrector was about 176 inches; with the corrector applied, it was 

 shortened by 24 inches. 



With reference to the driving-clock it is only necessary to say. that 

 its )-eguiator was a, heavy conical pendulum, or rather pair of pendu- 

 lums, weighing some 15 pounds, and so hung that tlieir revolutions 

 were sensibly isochronous through quite a range of inclination. 

 Whenever by increase of diiving power or decrease of resistance one 

 of the balls rose above a certain limit, it acted, without affecting the 

 radial motion of the ball, upon a friction spider which absorbed the 

 superfluous energy in the manner made familiar by the chronographs 

 constructed by the Clarks and by Fauth & Co., and now so common 

 in our observatories. The regulator revolved once a second. Tlie 

 gearing and driving screw were. constructed, for the most part, by Dr. 

 Draper himself, with the utmost care and accuracy ; and it may safely 

 be said that in its ultimate perfected condition the driving-clock was 

 as good as any in existence, keeping a star upon the slit for an hour at 

 a time w'hen near the meridian and not disturbed by changes of 

 refraction. 



In the course of the operations a great many forms of spectroscopic 

 apparatus were employed. At first, as has been mentioned, a quartz 

 prism was used, simply interposed in the path of the rays a few inches 



