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234 -THE NORTHERN MICROSCOPIST. 
more oculars will the objective stand before increase of amplifica- 
tion ceases to open out fresh detail. In the first case increase of 
illumination, with wide apertures, is very apparent, a halfinch 
of 90° would admit twice the amount of light that one of 60° would 
pass, the illumination depending upon the square of the numerical 
aperture. This fact has been proved to us over and over again 
during experiments on photo-micrography; an instance will illustrate 
it. Several weeks ago, the photographs on Plate IVa. were taken ; 
now the object remaining upon the stage and the illumination (an 
Argand gas lamp) unaltered, it was found that 4 minutes exposure 
was required with Tolles one-inch objective of 35° combined with 
the C eyepiece, and 5.5 minutes with a Zeiss B of 40° (really 38°) 
and the A ocular to obtain pictures of the same intensity. The 
amplification was 128 diameters in each case, and both plates were 
developed in the same bath. 
In the second case, the excess of aperture often comes in useful 
during practical work. One sometimes perceives in the field some 
object, or some minute structure not easily recognisable with the 
existing amplification—if there is no aperture to spare, a higher 
power eyepiece will be of but little use, the imperfect image will be 
magnified ; and but little idea will be gained of its true character. 
If, on the other hand, the aperture is not fully utilized by the 
shallowest ocular, deeper ones will continue to develop detail and 
be equivalent to working with a higher power objective of wider 
aperture. Let it be borne in mind, however, we are not attempting 
to argue for the use of excessively deep eyepieces, except for the 
purposes of testing objectives, or for numerical apertures exceeding 
1.0, It is our opinion that the 2” (A), the 1” (C), and the “5 (D) 
oculars are the most useful, and any objective to be employed in 
research should be capable of use with the half-inch eyepiece 
without the definition of being impaired. By this means, as we 
shall show hereafter, any degree of penetration and long working 
distance may be secured with objectives of wide aperture. 
Prof. Abbe has shown us in his paper of last year that under 
certain conditions wide apertures are more likely to afford us a 
correct image of objects than lower angles, and the P angulatum 
illustration on p. 462* is a strict confirmation of this view. 
Now, in considering the relationship which should exist between 
aperture and power in the microscope, working distance is one of 
the items not to be overlooked, and in a regular series of objectives 
all apertures should be possible for practical amplifications (within 
reasonable practical limits): it is certainly incorrect, as a principle, 
to increase the aperture of an objective in order that it may stand 
a deeper eyepiece, if at the same time any appreciable amount of 
SOLER IGE TM 000 DS 2 FSA CETL Se eR MED ROO OR ape IRR ge ECT TERE 
*J.R.M.S., August, 1881, 
ee  -—a S_eeweennn e 
