SELECTION OF A SERIES OF OBJECTIVES. 185 
It will be seen that there is a wide divergence between Mr. Nel- 
son’s and Prof. Abbe’s figures. For instance, for N.A. 0°65 Prof. 
Abbe suggests an objective of 1/8 in. and Mr. Nelson a 4/10 in. 
Lastly, we may give Dr. W. B. Carpenter’s views as expressed in 
his latest publication on the subject.* 
“The 1/8 in. is (according to the writer’s experience, which is 
confirmed by the theoretical deductions of Prof. Abbe) the lowest 
objective in which resolving power should be made the primary 
qualification,—the 1/6, 1/5, 1/4, and 4/10 in. being specially suited 
to kinds of biological work in which this is far less important than 
focal depth and dioptric precision. ‘This view is strengthened by 
the very important consideration that the resolving power given by 
wide aperture cannot be utilized, except by a method of illumination 
that causes light to pass through the object at an obliquity corre- 
sponding to that at which the most divergent rays enter the objec- 
tive. Now, although in the case of objects whose markings are 
only superficial such may not be productive of false appearances 
(though even this is scarcely conceivable), it must have that effect 
when the object is thick enough to have an internal structure; and 
the experience of all biological observers who have carried out the 
most delicate and difficult investigations is in accord, not only as to 
the advantage of direct illumination, but as to the deceptiveness of 
the appearances given by oblique, and the consequent danger of 
error in any inferences drawn from the latter. Thus, for example, 
the admirable researches of Strasburger, Fleming, Klein, and 
others upon the changes which take place in cell-nuclei during their 
subdivision can only be followed and verified (as the writer can 
personally testify) by examination of these objects under axial 
illumination, with objectives of an angle so moderate as to possess 
focal depth enough to follow the wonderful differentiation of com- 
ponent parts brought out by staining processes through their whole 
thickness. 
The most perfect objectives for the ordinary purposes of scien- 
tific research, therefore, will be obviously those which combine 
exact definition and flatness of field with the widest aperture that 
can be given without an inconvenient reduction of working distance 
and loss of the degree of focal depth suitable to the work on which 
they are respectively to be employed. ‘These last attributes are 
especially needed in the study of living and moving objects; and 
in the case of these, dry objectives are decidedly preferable to im- 
mersion, since the shifting of the slide which is requisite to enable 
the movement of the object to be followed is very apt to produce 
disarrangement of the interposed drop. And, owing to the solvent 
power which the essential oils employed for homogeneous immersion 
* ¢Encyclopzedia Britannica,’ 9th ed., xvi. (1883) pp. 269-70. 
