MICROSCOPY. 569 
Do not suppose that this is the only way to exceed 82° in bal- 
sam practically. The 100° 1 objective of four systems was on 
quite a different plan. It is, as to plan, described in the “ Monthly 
Microscopical Journal” for March, 1872. There the inner three 
systems have 130° as a dry objective, the front as applied to those 
three serving only to admit such a pencil to the dry objective con- 
stituted of the inner three. But the 4 tested by Dr. Woodward 
has, as to the inner three, an angle of 105° in air, and, as they are 
used, while the front has some, but slight, influence upon the pen- 
cil passing through it. 
And now, to sum up, referring to my article of March, 1872, 
“Monthly Microscopical Journal,” and diagrams. I will quote 
Fig. 1 and explanation as theory, antecedent to fact. The four- 
system objective being subsequently made and authoritatively re- 
substantiating the theory. (See “Monthly Microscopical Journal” 
for June, 1873; paper by Dr. Woodward giving the angle as 100 
in balsam.) Quoting again, as to the case of the three systems, 
same Journal, same page, referring to Fig. 2. “* What is intended 
is to increase the refraction of the convex surface of the front by 
sharper convexity, or higher refractive material, or both, to the 
extent necessary to make up for the diminution at the plane sur- 
face according to the refractive power of the medium” in which 
the front surface is immersed. Now the results, according to this 
Second case, are well attested for angles considerably above 
82° in hard balsam ; account of all which will appear in good time 
for support of my theory! i. e., the universally accepted theory. 
For balsam of refractive index the same as common crown or 
plate glass I sie with pleasure, show to you at any time that the 
angle of the 1, objective, tested in London, is at least 90°; and 
that i is the kind of balsam Mr. Wenham has constantly talked of, 
witness each of his criticisms on my claims. Every time 
alluded to the balsam index, he has declared it practically the same 
as crown glass. Of course, when hardened to resin, it may have 
higher refractive index and reduce the angle a little. Hence cer- 
tain discrepancies as to amount of angle above 82°. For this rea- 
Son, I have used the semi-cylinder, but that has, and had, another 
and a superior purpose. As a means of getting the actual angle, 
and the crucial test to decide this discussion, a much simpler 
method will serve. Thus, any piece of plate glass, say an inch 
Square or upwards, and perhaps zh inch thick, or more or less, one 
