ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
231 
are no edges and raphae by which false ghosts can be made ; it is 
specially a direct solid cone test, and an obliquo azimuthal beam is not 
of much assistance. This diatom is numbered in Moller’s 400 “ Typen- 
Platte” 3.3.11; the hoops are also common in “Sozodont,” and the 
striation will be found pretty constant at 60,000. 
In the second place, this correctional collar has another office. It is 
admitted that an iris diaphragm is exceedingly useful for the purpose, of 
regulating the size of the illuminating cone, but there is this objection 
to its employment, viz. that it is quite impossible to record the size of 
the hole or to reproduce a similar sized hole on a subsequent occasion 
with sufficient accuracy for microscopical purposes. A wheel of dia- 
phragms, or a series of graduated diaphragm discs to drop into a holder, 
are on this account recommended for critical work, because the dia- 
phragm can always be recorded, and the identical illuminating cone 
precisely reproduced at any subsequent time. 
The disadvantage with this plan is the loss of the power of delicate 
graduation between any two contiguous sizes of diaphragms, a want which 
the iris supplies. Now the correctional collar meets this deficiency by 
making the wheel of diaphragms as adjustable as the iris, in addition to 
securing an advantage which the iris does not possess, viz. that the 
size of the illuminating cone can be recorded, and with facility exactly 
reproduced at any time. For we know that by separatiug lenses the 
power of a combination is reduced, and conversely, by closing lenses it 
is increased, and also that with powers of equal apertures the size of the 
back lens must be larger with the lower power. Now, if we by means 
of the correctional collar separate the lenses, we shall reduce the power 
of the combination, and if the back lens, i. e. the diaphragm aperture, 
remains the same, the aperture of the combination will be lowered. 
Hence we have a very simple means of graduating the apertures 
between any two contiguous diaphragms ; if, for example, we place the 
lever to the left, so that the lenses may be separated as far as possible, 
and use a No. 6 diaphragm, and if on examining the object it is thought 
that the illuminating cone is not large enough, and if when No. 7 is 
turned on it is found too much, we can go back to No. 6, and by turning 
the lever 60° towards the right, closing the lenses and increasing the 
power a little, we shall obtain an aperture somewhere between the No. 6 
and 7 diaphragms (fig. 34). This position of the lever may be called 6^ ; 
if this is an insufficient cone we can turn the lever through another 60°, 
which will further close the lenses and increase both the power and the 
