300 
CORRESPONDENCE. 
means, cannot strike on tlie object at an angle greater than 41° if it is 
either in balsam or on tbe slide, but on this principle we are dealing 
with rays entirely 7 beyond this angle.” * 
If, by suitable adjustment of the mirror, we can ensure tbe illu- 
mination of tbe object solely with rays from tbe Illuminator of greater 
obliquity than 41° — which is admitted by tbe construction — tbe field 
rays must also be of greater obliquity than 41° ; and if, with pneumo- 
lenses these rays are necessarily lost by total reflexion at tbe cover- 
glass, whereas with certain immersion lenses a portion is refracted 
into a luminous field ; it follows that rays beyond tbe “ critical ” 
angle from balsam to air are “ got through ” tbe immersion lens, and 
Mr. Wenham’s dictum that “no object-glass can collect image-forming 
rays beyond this limit ” is confuted. 
With reference to tbe “ simple demonstration” cited in Mr. Wen- 
bam’s postscript for my special benefit, I observe be bad already 
favoured us with tbe method of procedure.! I try tbe experi- 
ment : — He says, “ Focus tbe top surface of tbe plate glass with an 
immersion -|tb having tbe highest available aperture;” be must here 
mean to request me to focus tbe immersion lens used as a dry lens, 
because later on be asks me to look sharply while tbe water inter- 
medium is applied. I take up Hartnack’s No. 9 immersion having 
tbe highest available aperture, and find it will not focus tbe definition 
with air as tbe intermedium : tbe experiment fails when tried with such 
an immersion lens ! But in order to meet him, I take Dallmeyer’s new 
^ in which tbe immersion front can be used wet or dry, and, adjusting 
it to tbe “ dry ” point, focus sharply tbe image of tbe line on the sur- 
face of tbe plate glass. I remove the ocular and allow tbe rays from 
tbe lamp-flame to traverse tbe optical body and tbe objective, they 
cross at tbe focus and form a luminous disk on tbe ground side of tbe 
plate glass ; tbe angular diameter of this disk is about 55°. I intro- 
duce tbe water film ; but it occurs to me I shall no longer be mea- 
suring tbe angle of tbe image-forming cone of rays,— tbe focus is no 
longer on tbe glass surface,— tbe lens must be adjusted for “ immer- 
sion.” As I do this, tbe angular aperture increases, and when I reach 
tbe point at which tbe image-forming rays are sharply focussed, tbe 
disk of light has increased to 70° ! With air as tbe intermedium, tbe 
image-forming aperture is 55° as shown by tbe disk of light ; with 
water it is 70°. But Mr. Wenbam asserts that whether there is water 
or air as tbe intermedium “ not tbe slightest change is visible in tbe 
diameter of tbe disk ” ! and in bis account of the experiment in 
No. lxiii., p. 116, be stated “ It made no difference in tbe angle 
whether water is admitted in front lens or not .... but in each case 
the object-glass must be focussed on the glass surface .” I must leave 
him to explain tbe discrepancy between bis result and mine. 
When Dr. Woodward forwarded Professor Keith’s diagram and 
trigonometrical computation to London in support of bis position, be 
wrote : ! “I cannot be expected to pay attention hereafter to tbe 
assertion of anyone who may continue to bold that it is ‘ theoretically 
impossible ’ to construct immersion objectives with a balsam aperture 
* ‘M, M. J.,’ No. xlii., p. 241. f Ibid., No. lxiii., p. 4! 6. 
I Ibid., No. lxix., p. 127. 
