320 Description of the Conurig-r/p Glass Telescope. 



of the eye-tube, each senii-leng forms an image of the same 

 object. These two images will be mofe of less separated 

 in the proportion of the distance of the centres of the semi- 

 lenses from each other, \vhich distance is shown in revolu-* 

 tions and parts of a revolution of st finger- screw that sepa- 

 rates theni. For this purpose a circular head is fixed oa 

 the linger-screw, th.e edge of which is divided into a hun- 

 dred ]i:uls ; and in order to know the number of fe^'olutions, 

 a small slip of brass (thra passes over the graduated surface 

 of the head, and serves as its index for showing the cente- 

 simal parts of a revolution) is fixed to the eye-tube, and 

 has its fljamfered edge also divided, each division being 

 equal to one entire revolution of the screw. 



To adjust the Telescope. 

 •rHavino-, by drawing out the eye-tube more or less, ad- 

 justed the telescope to distinct vision, turn the finger-scre^r 

 till the two images of the same object appear in one, and the 

 edae of the head together with the division numbered 1 00 

 will be found at the first division on the index that shows the 

 revolutions. 



The use of the Comivg-vp Glass. 

 Having directed the telescope to the vessel chased, turn 

 the finger-screw till the two images of some well defined part 

 of the vessel appear to have their extreme edges in contact 

 with each other ; then read oft' the number of revolutions 

 of the screw shown on the chamfered edge, »Iso (he parts of 

 a revolution shown on ih.e edge of the head: then if, after 

 some time, it be required to know whether we have gained 

 or lost in the chase, again bring the edges of the image* 

 of the same object in contact as before. If the number of 

 revolutions and parts of the snrew be the same as was showrt 

 before, we have neither gained nor lost in the ch.ase. But 

 if the number of revolutions and parts be less, the distance 

 from the chased vessel will be greater in the proportion of 

 the difference of these numbers to the former. On the 

 contrary, if the number of revolutions and parts be greater, 

 we come nearer the vessel in the prc^portion of the difference 

 of these numbers to the nuniber of revolutions and parts of 

 the fir-it observation. 



Exarnple. 

 Suppose in the chase of a vessel, by turning the finger- 

 screw, I bring the image of the main-top-yard to coincide 

 with the main- yard I and, reading off the value, 1 find it to 

 be three revolutions on the ehatniered edge, and 20 parts of 



a rcvo- 



