CORRESPONDENCE. 
267 
steady flames. These are now gradually separated till their minia- 
tures become indistinct. 
For a Eoss or Powell and Lealand stand, the arm may require a 
little lateral movement to bring the miniature flame into view. A 
better plan is to fix the objective to the stage by means of a clamping 
plate cut with the usual screw to receive the objective to be measured 
(front downwards). 
The angle contained between the centre and the two lamps will 
then give the useful angular aperture. 
Of course this method may be greatly refined by substituting 
illuminated patterns instead of the flames, or in front of them. 
A method sufficiently near may also be used by means of two 
lamps such as described, with only one objective. If the eye-piece 
be taken out, the miniature of the lamps will be seen down the tube, 
within the objective to be tested. 
Cover one lamp. Observe if the field on replacing the eye-piece is 
exactly one half illuminated. Next cover this lamp and uncover the 
other : move it if necessary till the other half of the field is similarly 
illuminated. Now view both lamps at once with the eye-piece in- 
serted as before. When the lamps are properly arranged a curious 
figure will be seen in the centre of the field, caused by the intersection 
of two adjacent circles of light by the dark field. These circles should 
exactly touch each other in the centre. Very similar results will be 
obtained by both methods. This plan has been tried many years ago. 
Opticians get a very fair idea of aperture by simply looking down an 
objective placed close to the eye, with its nose gradually turned from 
a lamp. The angle described at the point of vanishing of the light 
gives a very good rough estimation of the angle. 
I am yours faithfully, 
G. W. Eoyston-Pigott. 
P.S. — An aperture of nearly 180° can only be considered as an 
optical curiosity of no practical use. When, however, it is desirable to 
see markings developeable only by extremely oblique light, extremely 
high apertures doubtless show some appearances actually impossible 
of detection with a low aperture ; as, for instance, Nobert’s lines and 
other striations. In such cases the least aperture competent for the 
purpose is the most valuable, and a series of measurements of working 
angles for given objects is now very much wanted. I have found an 
Iris diaphragm (used now for about five years) placed between the 
objective and eye-piece at the nose of the microscope, and furnished 
with a lever movement, capable of giving interesting results for 
measurement of angle roughly. 
