192 Mr G. A. Berry on Focus of Concavo-Conex Lenses, [sess. 
note on the Focus of Concavo-Convex Lenses the 
Surfaces of which are of Equal Curvature. By 
George A. Berry, M.B., F.B.C.S. Ed. 
(Read December 18, 1893.) 
The ophthalmic surgeon has frequently to prescribe convex 
lenses of less than four inches focus for the correction of the 
optical defect in the eye left by removal of the intransparent 
crystalline lens in the operation for cataract. Occasionally, too, 
equally strong concave lenses are required to correct high degrees 
of short sight. 
Sometimes it is necessary to ascertain if the optician has ground 
a particular lens accurately according to prescription. The readiest 
method of doing this is to take a lens of opposite sign to, and same 
focus as, that prescribed, and observe whether or not, when the 
lenses are placed in contact, they neutralise each other. 
In the case of weak lenses (of more than four inches focus), a 
practical neutralisation is got when plus and minus glasses of equal 
strength are combined. For strong convex lenses, however, a 
stronger concave, and consequently for strong concave a weaker 
convex lens is required to produce the effect of neutralisation. 
The positive effect given by the combination of equally strong 
concave and convex lenses is more marked, and therefore appreci- 
able with weaker lenses, on putting the concave surface next 
the eye, than when the combined lenses are held in the Opposite 
way. 
The two lenses, plus and minus, of equal strength when held 
together so that one surface of each lens are in contact, are equiva- 
lent to a lens the two surfaces of which are of equal curvature, but 
one convex and the other concave. 
The properties of such a lens, inasmuch no doubt as it probably 
cannot be put to any use for which other forms of positive lenses 
are not more suitable, do not seem to have been studied. 
From the accompanying figures it is seen that F, or the distance 
