The Presidenfs Address. 
73 
sarily be impaired^ supposing the workmanship to be in all 
cases equally perfect. 
With a given objective, I believe the only available modes 
of increasing the magnifying power to be by increasing — ■ 
II. The length of the body ; 
III. The power of the eye-piece. 
In comparing these two modes of augmenting power_, my 
own observations are fully borne out by the concurrent tes- 
timony of several independent and able observers_, that with 
the same object and objective, under the same conditions of 
illumination, any required amount of amplification is ob- 
tained, with more light, and with far better definition, by elon- 
gating the body of the microscope, than by employing a deep 
eye-piece. 
It will doubtless be conceded as an axiom, that the most 
perfect view of any given object will be obtained by that 
amount of amplification'which is Just sufficient to distinguish 
and separate its smallest visible parts; beyond this point 
further amplification is only injurious, in rendering unavoida- 
ble instrumental imperfections more conspicuous. 
In using the V-^h objective and equivalent magnifying 
powers, I have tried every mode of illumination with which 
I am acquainted — the condensers of Gillett, Kingsley, 
Powell, and Ross ; objectives of all powers, from 3 inches to 
,Vrth ; and eye-pieces from A to F, and of various kinds. 
.Amongst all these contrivances, the most satisfactory efi^ects 
I have obtained in developing the" most minute visible points 
of structure in animal tissues, were produced with a Kelner C 
eye-piece, the focus of the eye-glass being a little below the 
object, and the light (direct light from a paraffin lamp, or 
argand gas burner) toned down by a diaphragm of suitable 
aperture placed immediately beneath the field-glass. 
With a magnifying power of about 3000 diameters,"^ one 
* The inagnifyinf? power in this instance, according to Dr. Beale's mode 
of estimation, would have been GOOO diameters; but this method of esti- 
mating the power employed is likely to produce some misapprehension. 
Dr. Beale compares the apparent magnitude of tiie distance between two 
conliginous fine lines (2000 or 5000 to the inch) of a ruled micrometer with 
the interval between the points of a pair of compasses placed the side, of 
ike stage^ irrespective of the distance of the stage from the eye. His 
alleged reason is that he is in the habit of making his drawings at about 
the same distance from the eye as the object i?, and therefore that his 
mode of estimating will correctly represent the amplification of the object 
in. his drmchig ; and this is doubtless perfectly correct. But in speaking of 
absolute magnifying power, it is necessary to compare the apparent magni- 
tude of the object with a known magnitude ])laced at a constant distance 
from 1-he eye, and for this purpov'ic a distance of ten inches is generaiiv 
adopted. 
