BuRTOX — On a Spectroscope of the Binocular Form. 43 



This property of each prism is made availahle by mounting it on 

 circular woodea disc, centrally perforated to admit a binding screw, 



Fis. 2. 



which secures it to the inner side of J5, and when tightened prevents 

 the disc from rotating. The discs and binding screws attached to the 

 prisms are shown at t t', N N'. 



The observer can readily turn round the wooden disc with the 

 attached prism, after slackening N, and he can fix the prism again in any 

 position he pleases by once more tightening iV. The two prisms are 

 perfectly independent of one another. 



Telescopes. The telescopes T T are fixed, like the collimators, to 

 a single mahogany base-board, and move also in a vertical plane about 

 the axis of the spindle a" a," which moves with the base-board. One 

 extremity of this axis, that marked a", is enlarged for the reception 

 of the clamp of the recording apparatus, to be mentioned presently. 

 The adapters, carrying the eye-pieces, are connected together by a pair 

 of cross bars l, V , which are secured to them by screws passing 

 through the slots in the outer tubes shown in the plan, 



Focussinc/ of eye-pieces. The focussing movement is communicated 

 to the eye-pieces simultaneously as follows. A piece of fiat linked 

 chain, c, is fastened at one extremity to the crossbar nearest the object 

 glasses, passed twice round the spindle W, which is fixed to the 

 wooden base plate, and secured by a clip arrangement to the cross 

 bar above the eye-pieces. On turning W, the chain is wound one 

 way or the other, and the adapters move with it. 



Each of the eye-pieces has an independent sliding motion in its 

 own adapter. In the focus of each eye-lens is a pointer, of a wedge 

 form, borne by a split tube, fitting modei'ately tight into the eye-piece 

 adapter. These pointers are capable of adjustment in every direc- 

 tion. 



