ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
671 
By the three fixed points in fig. 19 that error is eliminated. But to 
make three points which shall include accurately a given chord is a 
matter of some difficulty ; I therefore determined to simplify mine by 
Fig. 73. 
A, lens being measured ; B, ring ; C, drum-head ; D D, legs of tripod, 9 in. long ; 
E, glass plane surface ; F, lens for examination of Newton’s rings. 
dispensing wit‘h the three points altogether, and by substituting for them 
a ring. The ring is of course much cheaper to make, and that, too, with 
less liability to error. 
The screw is common to both, and the error on that score is the same 
in either case ; but with regard to accurate centering the ring probably 
has rather an advantage. 
There is a difference in the way of using them, for while the sphero- 
meter in fig. 19 rests on the lens, in this one the lens rests on the 
spherometer. 
My spherometer consists of a cylindrical ring supported on a tripod ; 
