34 ON THE CONSTRUCTION AND USE OF 



bonic Acid by Plants, the Fixed Lines of the Spectrum, Spectrum Analysis, &c., the 

 results of which are to be found scattered through the Philosophical Magazine, 

 Silliman's Journal, and the Journal of the Franklin Institute, he never pursued this 

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

 some with a five inch lens, driven by a heliostat. 



In 1850, Mr. Bond, taking advantage of the refractor of 15 inches aperture at 

 Cambridge, obtained some fine pictures of the moon, and subsequently of double 

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

 was also applied to move the telescope upon a polar axis. 



Besides these, several English and continental observers, Messrs. Hartnup, Phillips, 

 Crookes, Father Secchi, and others, have worked at this branch of astronomy, and, 

 since 1857, Mr. Lewis M. Rutherfurd, of New York, has taken many exquisite 

 lunar photographs, which compare favorably with foreign ones. 



But in none of these instances has the use of the sliding plate-holder been per- 

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

 pletely of the difficulties arising from the moon's motion in declination, and in the 

 second, instead of injuring the photograph by the tremors produced in moving the 

 whole heavy mass of a telescope weighing a ton or more, it only necessitates the 

 driving of an arrangement weighing scarcely an ounce. 



My first trials were with a frame to contain the sensitive plate, held only at three 

 points. Two of these were at the ends of screws to be turned by the hands, and 

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

 well in many respects, but it was found that however much care might be taken, 

 the hands always caused some tremor in the instrument. It was evident then 

 that the difficulty from friction which besets the movements of all such delicate 

 machinery, and causes jerking and starts, would have to be avoided in some other way. 



I next constructed a metal slide to run between 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 by one screw, worked better than the preceding. But 

 it was soon perceived that although the strips fitted the frame as tightly as practi- 

 cable, an adhesion of the slide took place first to one strip and then to the other, and 

 a sort of undulatory or vermicular progression resulted. The amount of deviation 

 from a rectilinear motion, though small, was enough to injure the photographs. 

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

 was called into active service, and I spent several months in Virginia. 



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

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

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

 presented me on my return with a slide and sand-clock, with which some excellent 

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

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

 ing the sliding frame 7t, 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 &, b'. 

 The cord i 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 whole steadiness, and yet 



