ON OPTICAL INSTRUMENTS. ♦ 435 



principle, and which seems to be somewhat less liable to error. In a reflect- 

 ing telescope of Cassegrain's construction, Mr. Ramsden has also pro- 

 duced the same effect by dividing the convex speculum, and causing a 

 part of it to turn round an axis. All these arrangements particularly 

 deserve the attention of those who are employed "n practical astro- 

 norriy and in geography, since the advancement of these sciences much 

 depends on the accuracy of the telescopic and microscopic measures, which 

 are performed by means of optical instruments, (Plate XXVIII. Fig. 

 416, 417.) 



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