Binocular Glasses for Eyes of unequal Focal Lengths. 461 
the salt and water a supersaturated solution differs in no 
respect from an ordinary solution. 
(4) The state of unstable equilibrium existing in a super- 
saturated solution is analogous to that existing in water cooled 
below its freezing-point. | 
(5) It is probable that full hydration of a dehydrated 
salt previous to solution occurs only when the salt is dis- 
solved in pure water or in a very dilute solution of itself. 
LII. On Binocular Glasses adjustable to Eyes having unequal 
Focal Lengths. By Colonel Matcoum, &.#., C.B.* 
ema many people are born with their eyes differ- 
ing in focal length or not I cannot say; itis sufficient 
that I have arrived at a period of life when glasses are neces- 
sary to enable me to read with comfort, and also that I found 
a very highly treasured pair of binocular glasses, by Voigt- 
Jaénder, become yearly more difficult to use, and at last useless ; 
but I found that I could see with equal distinctness with each 
eye, using only one eye at a time, on condition of a very slight 
alteration of focus. 
Mr. Browning, optician, in the Strand, having tested my 
eyes for a pair of pince-nez, told me that, as regards the 
ordinary reading and writing distances at any rate, my eyes 
were not a pair, and that after a certain age few people’s were 
who had used their eyes much. 
Accepting the fact, I set to work to make my binocular 
glasses once more useful—in this way. 
One tube is left untouched ; the eyepiece of the other is so 
arranged that it can be moved through a small range in and 
out, with reference to the eyepiece of the untouched tube, 
by turning round a milled ring. An index arrangement is 
rovided. 
The unaltered tube is used with one eye and brought to 
the most perfect focus possible in the ordinary way ; then the 
other tube is used with the other eye, and by means of the 
adjustment its definition is made as perfect as may be, the 
ordinary adjustment not being interfered with. The two eyes 
are then used together; and the process of adjustment had 
better be gone over again, as certainly the two eyes do help 
each other. 
The final position of the index-mark is noted; and that 
holds good for all ranges, as far as I have tried. 
Having noted this, you may lend your glasses to your friend, 
* Communicated by the Physical Society. The glasses were exhibited 
at the Meeting held on March 28, 1885. 
Phil. Mag. 8. 5. Vol. 19. No. 121. June 1885. 21 
