NOTES. 507 
together again at the point of leaving the back lens, and must 
. either leave it converging to some one fixed conjugate focus, or 
else parallel but not united; in the first case the combination 
could only be applicable to one fixed length of body, and in the 
other it would not be satisfactory under any conditions. The cure 
for this seemed to be, and proved to be, to transpose the single 
middle and the triple back; the over corrected triple bringing 
together the rays which had been separated by the single front, 
and tne single lens of longer focus making the rays parallel at the 
point ct final emergence. The single front is nearly alike in all 
cases, var ving only with the power required; the triple middle is 
of about -hree times, and the single plano-convex back four and 
ahalf times the radius of the front. The single plano-convex of 
long focus is reversed when transferred from the middle to the 
back position, the plane surface being above instead of below. 
Perfect color correction can be obtained by this formula in all 
screw collar objectives, from 4 inch upwards. This combination 
consists of five lenses and ten surfaces, taking the place of eight 
lenses with sixteen surfaces. 
These results are worked out by diagrams more easily than by 
mathematical computation ; the course of the rays being projected 
by means of proportional compasses, with surprising accuracy, 
on a scale of some fifty times the size of the real combination. 
Tories’ Triretets. — A correspondent writes as follows regard- 
ing a half inch triplet lately made by Mr. Tolles. “I am greatly 
pleased with the lens. Its performance is splendid, and it really 
gives the naturalist when away from his microscope an extraor- 
dinary facility. I should be very sorry to be without it.” We 
quote this from our friend’s letter, which was by no means de- 
signed for publication. These triplets certainly surpass anything 
of the kind we have met with. Mr. Tolles has just finished a 7, 
objective, which is perfectly satisfactory to himself. 
NOTES. | 
Ir is seldom that the sad record we are now obliged to make 
occurs in a single number of a magazine :—the loss by death of 
four valued contributors within so short a time. 
f. Jons Lewis Russext, of Salem, died on the 7th of June, . 
in the 65th year of his age. Prof. Russell was one of the founders, 
