120 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
5 in. The refractive power of a lens is the reciprocal of its focal 
length. Hence the equation is 
1 1 _ 1 
5 “ io “ To* 
Then refractive power of ocular + excess of ref. of eye == ref. power 
of eye and ocular taken as one, or 
11 G 1 
2 + To = To = “1-66 
Hence the resulting amplification will be as if the ocular had a focal 
length of 1*66 instead of 2, and the formula will be 
1 1 1 
p 10 “ 1-66 
1 1 1 11-66 
p ~ 1-66 lO “ 16-6 
16-6 
p ~ TT-66 
10 _ 1166 _ 7 
a ~ 16-6 ” 16-6 
11-66 
Hence for the observer whose eye is accommodated for 5 in., the 
expression for total amplification of the 1-in. objective and the 2-in. 
ocular will be 9 x 7 = 63. 
On the other hand, let us take a case much more unlikely to occur in 
practice, that of a person who by reason of excessive liypermetropia or 
paralysis of accommodation, is unable to focus any but parallel rays 
upon his retina, or, in other words, to accommodate for any point nearer 
than infinite distance. If such an observer make use of the same com- 
bination of objective, he has in effect subtracted from the refractive 
power of the ocular the refractive power of the lens, which represents 
the difference between a refractive power 0 and of 10 in. = 1/10 ; then 
1 _1_ _ _ 4 _ _ i 
2 "" 10 “ 10 2TT* 
Hence the resulting amplification will be as if the ocular had a focal 
length of 2 * 5 instead of 2, and the formula will be 
1 _ J_ 
V ~ 10 2-5 
1 10 1 125 1 
p ~ 25 + 10 ~ 250 ~ 2 
P = 2 
