Chromatic Carves of Microscope Objectives. By E. M. Nelson. 15 
As that part of the table on the fly-leaf of this Journal entitled 
“ Limit of Resolving Power ” is based on the assumption that the 
object is illuminated by a single beam of utmost obliquity in one 
azimuth, and as such illumination should only be used for the special 
resolution of very fine lines ruled on glass, such as Nobert’s or 
Fasoldt’s bands; further, as the resolution of a high number of lines 
with a single oblique beam in one azimuth is, as I have frequently 
pointed out, no criterion of the quality of an objective, because only 
the outer zone of the objective is utilized, the practical value of that 
part of the table is somewhat diminished. 
The following table is, however, constructed to meet the every-day 
wants of the practical microscopist. It gives the resolving power of 
first-class objectives with a 3/4 cone of direct illumination, both with 
white and blue light. For white light a line lower down the spectrum 
between D and E has been selected, because it more nearly represents 
the light which makes the strongest impression on the retina. For 
monochromatic light a line is taken a little higher up than F. It 
will be noticed that photography is also classed with this wave-length ; 
from practical experience I am convinced that, with ordinary photo- 
graphic methods, light near the line H has very little influence on a 
photomicrograph. Glass is not very transparent to rays of a short 
wave-length, and when we consider that the light has often to pass 
through, besides the slip, cover- glass, mounting medium, oil-immersion 
fluid, 19 lenses without counting the bull’s-eye (which I seldom use), 
it is not to he expected that an ultra-violet light should have much 
potency. 
The number of lines resolvable with a 3/4 cone of direct illumi- 
3 \ 
nation can be calculated by the formula (N.A.), A being the 
number of waves per inch. If we take, therefore, a wave-length of 
1/46,566 in. ( = 0*5443 f) for the white light, and of 1/53,333 in. 
( = 0*4762 f) for the monochromatic blue light, 3 X / 2 will equal 
70,000 and 80,000 respectively. All then we have to do is to 
multiply the (N.A.) by 70,000 for white light, and 80,000 for mono- 
chromatic blue light. 
Great accuracy is not needed in choosing the line, because the 
retina is not affected by a few hundred waves more or less, and pro- 
bably all persons are not influenced alike. It will be observed that 
the resolving powers for blue light are not carried on with apertures 
greater than 0 * 8 N.A. The reason for this is that its effect here 
ceases ; further, results obtained with higher apertures are hardly up to 
the values given under white light, even when blue-green light is used. 
In my former paper, already alluded to, the advantages derived 
from the use of monochromatic blue light were stated to be owing to 
the shortening of the wave-length ; this statement, although quite 
correct in theory, must not be pushed too far, especially with higher 
apertures. It makes a difference of about 14 per cent, in the case of 
low apertures, but beyond those of 0*9 N.A. its influence in increas- 
