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I alio made a computation, as accurately as I was able, of the 

 number of optical organs which were to be feen, in a row, in 

 the largell fcgment, or longell fide of the eye, and I found the 

 number amount to fixty ; and if we fuppofe that, in the fmalleft 

 fegment, or narrowed part of a Beetle's eye, there are forty fucli 

 optical organs in a row, thefe fixty and forty, added together, make 

 one hundred ; the half of which, being fifty, is what we mull take to 

 be the number of eyes in the hemifphere which compolcs a Beetle's 

 eye, if we confider fuch eye to be of a fpherical fliape. 



Now, if we fuppofe that the two eyes of a Beetle do, together, 

 make up a perfect fphere, and that in a great circle* on fuch fphere, 

 one hundred optical organs are placed, we fliall find, according 

 to this rule laid down by Metius, " As 22 is to 7, fo is the 

 " quadrature of the great cii"cle to the contents of the fuperficies," 

 and the rule being applied as follow^ : 



i* • la^ j > a 



18 



the refult is, that if each eye in a Beetle's head is taken as an hemi- 

 fphere, the two Mill make up a fphere containing, on its furface, 

 3181 optical organs or eyes. 



I have caufed a drawing to be made of the eye, or rather of the 



* It has been mentioned in another place, but is here repeated to favc the trouble of 

 turning back, that by the great circle is meant, the largcft which can be drawn upon a globe, 

 or fphere, or, in other words, a circle, M-hofc diameter is a line paffing through the centre of 

 fuch fphere to the furface. 



