40 O N T HE CONS T R U C 1' I O N A N D USE OF 



ments in the rod f, to produce e(|ual changes in the flow. Tlie rating requires 

 consequently only a few moments. 



The object of the side tube /is to avoid disturl)ing </ when it beconles necessary 

 to refill the cylindcn-, for when it is once opem^d to the right degree, it hardly 

 requires to l)e touched again during a night's work. In ordcn- to arn^st the down- 

 ward motion of tlu' piston at any point, a cLuup screws on the piston rod, and can 

 be brought into contact witli the cyUnder head, as hi the figure. 



That this instrument sliould operate in tlie b(>st manner, it is essential to have 

 the interior of the brass cylinder polished from end to I'ud, and of uniform diameter. 

 If any irregularity shoidd be percei\ed in the rate of going, it can be cured com- 

 pletely by taking out the piston, impregnating its leatlier stuffing with fine rotten 

 stone and oil, and then rubbing it up and down for five nunutes in the cylinder, so 

 as to restore the polish. The piston and cylinder nuist of course be wiped, and 

 regreased with a mixture of beeswax and olive oil (cMpial i)arts) after such an 

 operation. In replacing the piston, the cylinder nnist be first filled with water, to 

 avoid the presence of air, which woidd act as a spring. 



Although it may be objected that this contrivance seems to be very troublesome^ 

 to use, yet that is not the case in practice. Even if it Avere, it so far siirpasses any 

 prime mover that I have seen, where the utmost accuracy is needeil, tliat it would 

 be well worth employing. 



c. The S'lm Camera. 



In taking ))liotographs of the sun with tlie full aperture of tliis telescope, no 

 driving meclianism is necessary. ( )u the contrary, th(" difficidty is rather to arrange 

 the apparatus so tluit an exposure short enough may be given to the sensitive plate, 

 and solarization of the picture avoicU-d. It is not desirable to rtnluce the aperture, 

 for then the separating power is lessened. Tlie time reciuired to oljtain a negative 

 is a very small fraction of a second, f(U' the wa\y appearance produced by atmos- 

 pheric disturbance is not luifreiiuently observed sharply defined in the photograph, 

 though these aerial motions are so rapid that they can scarcely be counted. Some 

 kind of shutter that can admit and cut off the solar image with great quickness is 

 therefore necessary. 



in front of an ordinary camera rr, Fig. 36, attached to the eyepiece holder of the 



telescope, and from wliich tlie lenses have been remoxcd, a spring shutter is fixed. 



It consists of a quadrant of thin wood />, fastened by its right 



Fig. 36. '■ . . 



angle to one corner of the camera. Over the hole in this 



([uadrant a plate of tin d can be adjusted, and held in position 



by a screw moving in a slot so as to reduce the hole if desired 



to a mere slit. It may vary from Ig inch to less than J^ of 



an inch. The quadrant is drawn downwards by an India- 



rul)ber spring g, 1 inch wide, | of an inch thick, and 8 inches 



long. This spring is stretched when in action to about 12 



The Spring Shutter. inches, aud wheu released draws the slit past the aperture c in 



the camera. Two nicks in the edge of the quadrant serve with the assistance of a 



pin e, which can easily be dra^vn out by a lever (not shown in the cut), to confine 



