34 ON THE CONSTRUCTION AND USE OF 



bonic Acid l)y Plants, the Fixrd Lines of the Spectrnm, Spectrum Analysis, &c., the 

 results of Avliicli are to he fouud scattered tlirough the Philosophical INIagazine, 

 Silliniau's Journal, and thi' .lourual of the Franklin Institute, he never pursued this 

 very promising subject. Souu^ of the pictures were taken with a three inch, and 

 some Avitli a live inch lens, driven by a heliostat. 



In l.SoO, INIr. Bond, taking advantage of the refractor of lo inclics aperture at 

 Cambridge, olitained some line pictures of tlie moon, and sul>s((|uentl\ ot double 

 stars, more particularly Mizar in Ursa Major. Tlie driving power, in this instance, 

 was also applied to move the telescope upon a ])olar axis. 



Besides these, several English and continental oliservers, Messrs. llartimp. I'liillips, 

 Crookes, Father Secchi, and others, liave worked at this l)r;inch of aNfroiioniy, and, 

 since 1857, Mr. Lewis M. llutherfurd, of New \'ork. has taken nuiny exquisite 

 lunar photographs, which compare fivoral)ly witii Ibreign ones. 



But in none of these instances has the u.se of the slidhig plati-liolder been })er- 

 sisted in, and its advantages brought into view. In the first place it gets rid com- 

 pletely of tlie difficulties arising from tlie moons motion in di'dination. and in the 

 second, instead of iiijnring tlie photograph b\ ihc tremors produced in mo\ing the 

 whole lunivy njass of a teles('ope weigliing a ion or more, it only neci'ssitatt's the 

 driving of an arrangement weighing scarcely an onnce. 



Mv first trials were with a frame to contain the sensiti\e plate, held oidy af lliree 

 points. Two of thes(» were at the ends of screws to lie liirned by the hands, and 

 the third was on a spring so as to maintain firm contact. This apparatus worked 

 well in nnuiv res])ects, bnt it was found tliat liowever mucli care might b(> taken, 

 till' hands always caused some tremor in the instrument. it was e\ident then 

 that the difficulty from friction \\hicli besets the movements of all such (hdicate 

 machinery, and causes jerking and starts, wou id ha\c to lie ;i\ oidcd in some other way. 



I next constructed a metal slide to run itetween two parallel strips, and ground 

 it into position with the greatest care. This, when set in the direction of the moon's 

 apparent path, and moved liy one screw, worked better than the preceding. But 

 it was soon perceived that althougli the stri])s fitted the tVame as tightly as practi- 

 cable, an adhesion of the slide took place first to one stri]i and tiien to the other, and 

 a sort of undulatory or vernncular ])rogression resulted, 'i'he aiuount of deviation 

 from a rectilinear motion, though small, was enough lo injmc the photographs. 

 At this stage of the investigation the regiment of vohniteers to which I belonged 

 was called into active service, and I spent several nuintiis iu ^'irginia. 



My Ijrother, !Mr. Daniel Draper, to whose mechanical ingenuity I have on several 

 occasions been indebted for assistance in the manifold difficidties that have arisen 

 Avhile constructing this telescope, continued these experiments at intervals. He 

 presented me on my return with a slide and sand-clock, A\ith ^\hich some excellent 

 photographs have been taken. He had found that miless the slide above mentioned 

 was made ungovernably long, the same trouble contiiuied. He then ceased catch- 

 ing the sliding frame h. Fig. 32, by two opposite sides, and made it run along a 

 single steel rod a, being attached by means of two perforated plates of brass h, //. 

 The cord v- going to the sand-clock, was applied so as to pull as nearly as possible in 

 the direction of the rod. A piece of cork c, gave the Avhole steadiness, and yet 



