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XIII. 



A NEW FORM OF DIPLEIDOSCOPE. 



By SIE HOWARD GRUBB, F.R.S., Vice-President, Royal Dublin 



Society. 



(Plate X.) 



[Read, November 17 ; Received for Publication, November 20, 1903 ; 

 Published, February 10, 1904.] 



Even in these days of rapid travelling and telegraphic communica- 

 tion there sometimes exists a difficulty in ascertaining true time 

 in country-places. 



The very causes which operate to render the distribution of 

 time more easy also necessitate a greater accuracy than was 

 necessary in the old coaching days. 



The sun-dial, therefore, is now valued more for its old-time 

 picturesque appearance than for its utility. We have practically 

 nothing available between this and the astronomical transit-circle, 

 which instrument is altogether too delicate to entrust to any but 

 highly-skilled hands. 



Many years ago I found a description of an ingenious instru- 

 ment called a " Dipleidoscope," 1 which consisted of a right-angled 

 prism fixed in such a position that the Sun when near the meridian 

 could be viewed in it obliquely. Two images were seen — a faint 

 one, due to the partial reflection from the first surface of prism, 

 and another very brilliant, being doubly reflected from the two 

 inner surfaces. 



As one of these images was due to single reflection, and the 

 other to double reflection, they appeared to move in opposite 

 directions ; and if the prism was properly set, the two images 

 overlapped at the moment the Sun passed the meridian. 



The instrument was not very effective, because, firstly, it was 

 necessary to view the • images through a very densely-coloured 

 glass to render bearable the intensely brilliant image formed by 



1 " A Description of tbe Dipleidoscope." By E. J. Dent, 1843. 



