NOTES AND MEMOEANDA. 
31 
and with results quite surprising. While studying high-power objec- 
tives of large angular aperture, X experimented with various methods 
of illumination, and among others applied the reflex illuminator. I 
find that some immersion objectives are capable of transmitting the 
extremely oblique rays that pass through the illuminator, so as to 
give a bright field when used on balsam slides. In dry mounts the 
light cannot be transmitted beyond the upper surface of the slide, but 
in balsam-mounted slides the light passes to the upper surface of the 
cover and is there totally reflected. If an immersion objective is 
adjusted and connected with the cover by a film of water, the total 
reflexion will be destroyed, and the light pass through the cover and 
water into the front of the objective. The ultimate direction of the 
ray of light after passing through the illuminator is not changed by 
the introduction of the diflerent media (balsam, glass, and water), and 
the angle at which it enters the objective must therefore be greater 
than 41°. In examining Holler's Probe-Platte, a balsam mount, under 
these conditions, with light from a kerosene hand-lamp, I easily 
resolved the Amphipleura pellucida ; so clear and decided were the 
lines that with a power of 8000 they were still visible. I first obtained 
this unexpected result with my Powell and Lealand Yt^h, and the light 
was sufficiently bright to render it possible to use a i-inch eye-piece 
and concave amplifier. As the Ytth. has the power of a -^-^th, I 
obtained, by estimation, a power of 80 00 diameters. 
The resolution of this difficult diatom, as well as the Frustulia 
Saxonica and NitzscMa curvula (Nos. 18 and 19 on the Probe-Platte), 
far surpasses any that I have ever seen by artificial light, and rivals 
the beautiful resolution obtained by monochromatic sunlight. With 
this illuminator it is much easier to resolve the Amphipleura in balsam 
than to resolve it dry with any other artificial illumination. 
I find, however, as yet but few objectives capable of transmitting 
light of such extreme obliquity through the back systems. The 
Powell and Lealand Y^th, as I have above stated, succeeds admirably. 
My Tolles' Ytth immersion gave only a dark field. 
Of several others I have succeeded with only three : a y\yth made 
several years ago, a four-system ^th, and a four-system y Q-th, the last two 
of recent construction, and all three made by Tolles, and all of course 
immersion. I have not had access to Continental objectives of wide 
angle. The three objectives of Tolles last named resolve the whole 
of the frustule of the Amphipleura at once, while the Powell and 
Lealand y^^th resolves only a part at one view, even when the whole 
frustule is in the field. The advantages of the reflex illuminator in 
thus furnishing light of greater obliquity than has been obtained by 
other methods, seem to me worth considering by those interested in 
testing the resolving power of objectives. 
I find it advantageous to connect the illuminator with the slide by 
glycerine, instead of water, as it does not evaporate. The higher 
refractive power of glycerine makes no difference in the ultimate 
direction of the light. 
With high amplification the lines of the Amphipleura become 
decidedly beaded, but do not separate into dots. 
VOL. XiV. D 
