00 
COBRESPONDENCE. 
means, cannot strike on the object at an angle greater than 41° if it is 
either in balsam or on the slide, but on this principle we are dealing 
with rays entirely beyond this angle." * 
If, by suitable adjustment of the mirror, we can ensure the illu- 
mination of the object solely with rays from the Illuminator of greater 
obliquity than 41°^ — which is admitted by the construction — the field 
rays must also be of greater obliquity than 41° ; and if, with pneumo- 
lenses these rays are necessarily lost by total reflexion at the cover- 
glass, whereas with certain immersion lenses a portion is refracted 
into a luminous field ; it follows that rays beyond the " critical " 
angle from balsam to air are " got through " the immersion lens, and 
Mr. Wenham's dictum that " no object-glass can collect image-forming 
rays beyond this limit " is confuted. 
With reference to the " simple demonstration " cited in Mr. Wen- 
ham's postscript for my special benefit, I observe he had already 
favoured us with the method of procedure. | I try the experi- 
ment : — He says, " Focus the top surface of the plate glass with an 
immersion -|th having the highest available aperture ; " he must here 
mean to request me to focus the immersion lens used as a dry lens, 
because later on he asks me to look sharply while the water inter- 
medium is applied. I take up Hartnack's No. 9 immersion having 
the highest available aperture, and find it will not focus the definition 
with air as the intermedium : the experiment fails when tried with such 
an immersion lens ! But in order to meet him, I take Dallmeyer's new 
^ in which the immersion front can be used wet or dry, and, adjusting 
it to the " dry " point, focus sharply the image of the line on the sur- 
face of the plate glass. I remove the ocular and allow the rays from 
the lamp-flame to traverse the optical body and the objective, they 
cross at the focus and form a luminous disk on the ground side of the 
plate glass ; the angular diameter of this disk is about 55°. I intro- 
duce the water film ; but it occurs to me I shall no longer be mea- 
suring the angle of the image-forming cone of rays, — the focus is no 
longer on the glass surface, — the lens must be adjusted for " immer- 
sion." As I do this, the angular aperture increases, and when I reach 
the point at which the image-forming rays are sharply focussed, the 
disk of light has increased to 70° ! With air as the intermedium, the 
image-forming aperture is 55° as shown by the disk of light ; with 
water it is 70°. But Mr. Wenham asserts that whether there is water 
or air as the intermedium " not the slightest change is visible in the 
diameter of the disk " ! and in his account of the experiment in 
No. Ixiii., p. 116, he stated " It made no difference in the angle 
whether water is admitted in front lens or not . ... hut in each case 
the object-glass must he focussed on the glass surface.'' I must leave 
him to explain the discrepancy between his result and mine. 
When Dr. Woodward forwarded Professor Keith's diagram and 
» trigonometrical computation to London in support of his position, he 
wrote : J "I cannot be expected to pay attention hereafter to the 
assertion of anyone who may continue to hold that it is ' theoretically 
impossible ' to construct immersion objectives with a balsam aperture 
♦ 'M. M. J./ No. xlii., p. 241. f Ibid., No. Ixiii., p. 116. 
X rbid, No. Ixix., p. 127. 
