64 W. J. M. Rankine on the Mechanical Action of Heat. 
As the Professor possesses several higher objectives, by Mr. 
Spencer, he has had executed by Mr. Grunow, at the same time 
with the Binocular, only an object-glass of 1 inch focus with 
which alone we have had an opportunity of trying the perform- 
ce. 
This object-glass has a large angle and is corrected exquisitely 
for chromatic and spherical aberration. That difficult point in 
microscopy, a view into deep cavities, is perfectly attained. The 
eye of a fine needle by incident light, exhibited the walls of a 
cavity deeper than broad; an opéned anther cell preserved in — 
fluid and happening to lie edgewise, exhibited by transmitted light 
its walls with clear vision to the bottom of the depth; the fibres 
of the fringed end of a silk ribbon floated in space in different — 
planes of superposition with an enlargement of distance in the 
perpendicular direction strikingly correspondent to their horizon- 
tal separation. No doubling or thickening of lines, mistiness, 
fog or uncertainty accompanied the views, but vision was brilliant 
and clear, while thus looking into space instead of being as it 
were confined by an optical wall of limitation, to a single plane. _ 
Neither while thus looking far into space by magnifying distance 
in proportion to surface, was any thing lost in power of minute 
discrimination of lines and edges; these were sharply and clearly 
defined: in short true shape and form were perceived in theif 
actual proportions instead of being flattened, and an entire whole 
seen, as Schacht expresses it, ‘in optical sections ;” and were 
exhibited simultaneously with minuteness of detail on surfaces. 
and the power of enormously enlarging the aperture of the. ob 
jective. 
New Haven, March, 1854. 

— 

Art. [X.—Mechanical Action of Heat; by W. J. Macquor’ 
ANKINE. 
Gentlemen—I beg leave to address to you the following Te 
marks on a formula referred to in the very able and interesting 
paper of Professor Frederick A. P. Barnard on “Heated Air con-— 
sidered as a Motive Power,” published in the American Jo ral 
of Science for March. 
The formula in question represents the maximum efficiency of 
a perfect 'Thermo-dynamic Engine: that is to say, the greatest 
fractional portion of the total heat consumed which such an el 
gine converts into motive power ; and, in Prof. Barnard’s notatiod, 
it is as follows : 


