198 Extracts from Mr. H. E. Fripps Translation of 
magnifying power reaches even to 4000, so will there be none in 
the future. On the contrary, the facts just stated show that 
amplifications of less than half 4000 — such as are readily obtained 
with objectives of inch, and seem really serviceable — are never- 
theless 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 amplifi- 
cation, but rather at a more correct performance of the middle 
and moderately high powers. It will 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 
ith and -|th what is now only attained with much higher powers. 
Such an aim is within the range of what is 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 
objective 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 jplane, 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 
(1 micro-mm. = 2^50-00- inch). Covering glasses of various thicknesses 
(accurately measured) are ruled on their under surfaces with lines 
2^0 to T2V0 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 
course of any pencil entering within the angle of aperture of the 
objective to be tested. 
The testing process has for its aim to view the co-operation of 
every zone of the aperture, whether central or peripheral, and yet, 
