568 MICROSCOPY. 
angle of (close to) 180°, after the first refraction, was necessarily 
reduced to 82° (closely) by crown glass plane surface, and by 
heavy, flint plane surface to 76° (closely). That is and has been 
understood, all around; though produced and constantly reit- 
erated as an answer to my claims, not only by Mr. Wenham, but 
volunteered with much rudeness from another quarter. 
ou comprehend the case perfectly when you say, “ This reason- 
ing assumes only that the extreme ray above the front combination, 
capable of entering into the image when the objective is worked 
dry, is the extreme also when adjusted for immersion work.” But 
it would be equally true to say, “the extreme ray above the front” 
surface “is also the extreme ray, etc.” In the light of this state- 
ment, what is to be understood by my March paper (Monthly Mi- 
croscopical Journal, 1873) to which you allude as ‘‘ practically 
disclaiming this doctrine of rays beyond the extreme rays dr pa” 
Why, I suggest the one sure way of giving entrance from the 
denser medium into the Front of a larger pencil than before with 
crown glass, in just so far as the refraction of the Front in such 
medium approaches the refraction of crown glass in air; and, 
behold! I am made to disclaim the very thing I have just done and 
pointed out how. However, from what you have written I know 
you will understand this:— my respondents say at once, “82° 
impinging on the inner front surface of the front lens will, from 
crown glass, emerge into the balsam without sensible deviation.” 
Now, suppose we use flint glass; the angle at which total reflec- 
tion takes place in this, when in contact with air, is not 82° (-)> 
but 76° (about). When a pencil of 82°, however, impinges upon 
this plane surface of flint, in contact with balsam, it will have pos- 
itive refraction according to the refractive index of flint glass 
in balsam and therefore while only 82° in the glass of the flint front, 
whether emergent or immergent would have more than that angle 
in the balsam. This much at least is sure and is decisive of the 
question. But again, if the material of the front surface have à 
refraction in balsam equal to that of crown glass in air, then ob- 
viously we might have near to 180° in the balsam, while the trans- 
mitted pencil immediately above the front surface would remain 
about the same as the “limiting angle” of crown glass in air, viz.: 
82.° This is valid principle and reasoning, but I cannot appeal to 
facts in this case. The best I have done is 112° in balsam. See 
“Monthly Microscopical Journal ” for June, 1873. 
