ZOOLOGY AND BOTANY. MICROSCOPY, ETC. 
359 
lines VOX and WOU indicate tlie paths through the lenses which the 
boundary rays of the light transmitted by the diaphragm ojiening at 4 
travelled. Experimentally it was found that a circular opening 2 mm. 
in diameter in a diaphragm capping the substage condenser at 2, a 
circular opening 3 mm. in diameter in a diaphragm capping the objective 
at 3, and a circular opening 5 mm. in diameter in a diaphragm placed 
behind the objective at 5, just permitted all the rays transmitted by the 
diaphragm opening “ 4 ” to reach the emitting surface at d. 
Fig. 47. 
Then, without otherwise changing the arrangement, for diaphragm 5 
with its opening 5 mm. in diameter was substituted diaphragm 8 (shown 
just behind 5), which had a zonular opening and an opaque central 
portion 8 mm. in diameter. Diaphragm 8 obstructed all the primary 
rays emitted by the lens at d. The eye-piece was then removed. On 
looking at the back of the objective, the zone c d uncovered by dia- 
phragm 8 was illuminated, and remained illuminated even when the 
circular opening at 4 was changed to “ 1 ” of the substage diaphragm. 
The peripheral zone at the back of the objective was illuminated by 
rays which must have been separated from the direct axial rays at a 
previously operative lens-surface. 
The 2-in. eye-piece was replaced ; the eye-lens of the eye-piece w r as 
removed, and a photomicrograph taken, as shown in photo 14. 
Diaphragm 5 was then exchanged for diaphragm 8, and the photo- 
micrograph taken with the same camera arrangement is shown in photo 17. 
Immediately afterwards, while observing the image of the annulus on the 
ground glass as the camera with its 1-in. objective w T as pushed slowdy 
towards the Microscope, the annulus was seen to shrink gradually and 
become the small cross shown in photo 18, which is inverted as to the 
larger cross in photo 14. There can be no doubt then that the annulus 
was illuminated by light which in some way was derived from the aerial 
image of the cross projected in the plane of the stage, and the inversion 
of the smaller image shows that the excentric rays were conveyed to at 
least one more focus than the direct axial rays. It is probable that the 
excentric rays were separated from the direct axial pencil by internal 
and converging reflection at the emitting surface of the front lens of the 
objective, and returned to the front surface, thence to be reflected back in 
an onward direction to and through the emitting surface along excentric 
paths towards the annulus. 
The Abbe “ spectra ” now claim our attention. We have found that 
they are not indispensable in some of the images of microscopic vision. 
“ Spectra ” are images of an opening in the diaphragm of a substage 
condenser, or of a source of light, formed above the objective in micro- 
