98 S. P. Langley — Observation of Sadden Phenomena. 



divided into five parts, so that the whole circle is divided into 

 twenty. All the observer has to say is in which quadrant and 

 in which subdivision of the quadrant the star appears, to say in 

 which twentieth of the second (or other brief cycle) it 

 emerged. 



The reticule I have just described is fixed in the focus of the 

 eye-piece and does not revolve. What does revolve is a minute 

 double prism of total reflection just before the reticule, the 

 middle of whose reflecting face lies in the optical axis, and by 

 whose means this axis is twice broken at a right angle, so 

 that when the telescope is directed on a star the image of 

 the star is not seen at the center of the field, but on one side of 

 it. If the prism is revolved, the star must appear to revolve 

 in a circle whose radius is nearly that of the side of the prism. 



The little prism is turned by a small piece of watch-work, 

 but it is not at all necessary that this should be exact, since all 

 we demand is that the rate shall be constant during a second 

 or so — a condition easily secured with the most ordinary 

 mechanism. 



The sketch and the apparatus exhibited sufficiently indicate, 

 I think, the simple means by which this is brought about. 



Lojrtp. 



Fig. 1. Section. 



& FullSizi 



Fig. 2. Diaphragm. 



Fuu. Size. 



S. P. Langley. invt 



Figure 1 is a section one-half full size. A A is the outer 

 tube, which can be fitted, if desired, into the eye end of a tele- 



