TRANS. OF PROF. ABBE’S PAPER ON THE MICROSCOPE. I21 
microscope, beyond what is attainable at present with objectives of 
zs inch focal length, is to be expected in the future, either by 
shortening of focus or by further improvement of construction. 
And as there exists at this moment no microscope whose serviceable 
magnifying power reaches even to 4000, so will there be none in 
the future. On the contrary, the facts just stated show that ampli- 
fications of less than half 4000—such as are readily obtained with 
objectives of ;4; inch, and seem really servzceable—are nevertheless 
not available in practice. The final inference from these data is 
that improvement of the microscope should no longer be sought 
for by aiming at still higher magnifying power and amplification, 
but rather at a more correct performance of the middle and mode- 
rately high powers.  J¢ wll be a real advance of the optician’s art, 
and of infinite service to the scientific use of the microscope, when 
we succeed in accomplishing with objectives of 1th and $th what ts 
now only attained with much higher powers. Such an aim ts within 
the range of what ts possible. 
In the account of Professor Abbe’s researches, to be hereafter 
published, new and exact methods will be given by which every 
determinable point in the construction of the microscope, e. g. focal 
length of each lens, angle of aperture, character and limits of objec- 
tive and ocular functions may be empirically ascertained ; and, in 
addition to this, a mode of procedure described which renders it 
possible, with very simple means, to examine in instruments already 
made, every fault of definition of image, and thus to determine 
their relative excellence. The methods commonly recommended 
for testing the state of spherical and chromatic correction of the 
objective are not adequate to the actual requirements of the case, 
and quite fail to explain the true character of the aberrations. 
‘The principle upon which the mode of proceeding to which 
reference has been made above, may be here generally indicated. 
As _ test-object, a preparation is used which presents only sharply 
outlined black and white lines alternating with each other, and 
lying in the same plane, so that no deviation can occur in the course 
of the rays transmitted through it. A preparation of this kind, 
sufficiently perfect for all practical purposes, may be made by ruling 
groups of lines, with the aid of a dividing machine, on the metallic 
film of silver or gold fixed by known methods on glass, and having 
no greater thickness than a fractional part of a micro-millimeter 
(r micro-mm. =;;4,5, inch). Covering glasses of various thicknesses 
(accurately measured) are ruled on their under surfaces with lines 
zo tO ,y55 to the inch, and cemented on a glass slide with 
balsam, one beside the other. A preparation of this kind serves 
for the highest as well as lowest powers. The illumination must 
be such that light may be reflected simultaneously from several 
sides upon the object, and means provided for regulating at will the 
