﻿486 . APPENDIX. 



Those to the 31st of March 17 88, were observed 

 with a glass made by lVatkins> that magnified abouc 

 no times; those from thence to the 12th of May 1 790, 

 were observed with one of Rmnsdeiis telescopes of the 

 sore lately made for the navy; and the remainder with 

 a glass made by Do!urad> that magnifies about eighty 

 times, 



I shall conclude these observations with a remark 

 f bat highly concerns both the buyers and makers of 

 telescopes ; namely, that the parts which compose the 

 object glass of an Achromatic, are generally put toge- 

 ther in such a manner that they cannot be taken asun- 

 der ; and the brass part that they are bedded in, shoots 

 a number of chymical ramification^ between the glasses, 

 that in the course of a year renders a telescope of little 

 or no service. This defect the maker may easily re- 

 move by making the compound object glass capable 

 of being taken to pieces, or the parts in some other 

 substance not liable to this defect. 



