1868.] 



Solar Eclipse of 1868. 



105 



thus uniform ; and though there is very perceptible irregularity in the actual 

 motion, it is not intermittent. Thus, when the image oi' a star, for instance, 

 is brought on the slit of the spectrum-apparatus, the spectrum is fitful in 

 appearance, if the slit is perpendicular to the direction of diurnal motion. 

 The mean motion may be easily regulated as in a pendulum-clock. The 

 motion is communicated by friction to the first of a series of wheels which 

 terminates in an endless screw working in the circumference of a large 

 toothed arc attached to the hour-axis. Motion imparted by hand to one 

 of these wheels, grooved and provided for this purpose with an endless cord, 

 is thus communicated directly to the tube without greater strain on the 

 clock than is implied in overcoming the connecting friction. 



3. Its Mounting. 



The declination-axis terminates in a T-shaped head carrying two circular 

 collars, in which the telescope-tube rests. For convenience in mounting 

 and dismounting, these collars are attached to the T-head by nut and pins, 

 so that they lift off with the tube, while the balance can be adjusted by 

 releasing their grasp of the tube when required. This is a great conveni- 

 ence in a portable instrument. The tube can be dismounted and taken 

 indoors readily without assistance ; and the body of the instrument (which, 

 besides being far less easily handled, has cost hours of adjustment) may be 

 left under a suitable waterproof case when no observatory has been con- 

 structed. 



4. Its Stand. 



The stand is a strong wooden one, of remarkably firm construction, consi- 

 dering that it is of the three-legged portable kind. Its upper surface 

 is a stout brass annulus, on which the clock-chamber rests and rotates, if 

 required, for adjustment in azimuth. Two of the legs have foot-screws for 

 adjusting the level and completing the adjustment for latitude. 



5. Of the Spectroscope. 



The spectroscope intended for use with the above telescope was con- 

 structed by Messrs. Simms, on a pattern or design supplied (I believe) by 

 Mr. Huggins ; but its construction was too much delayed to allow of a 

 practical examination of all its parts before packing. It consists of a 

 single flint-glass prism, of refracting angle 60°, contained in a cylin- 

 drical brass chamber, from which radiate three tubes in such directions 

 as to fulfil the several purposes of (1) receiving the light to be analyzed, 

 (2) delivering it after refraction and separation to the eye, and (3) 

 admitting external light for reflection to the eye off the second surface 

 of the prism. The first consists externally of a long connecting tube for 

 insertion into the telescope in place of the ordinary eye-tube, where it is 

 grasped in the focusing- slide. Internally it carries a smaller tube, carrying at 

 one end a lens, and at the other, at the principal focal distance of the latter, 

 a beautiful piece of workmanship by which a slit is obtained whose sides 



i 2 



