22 On a Modification of the " Slit for Testing Angle. 
slit. (Also, a central portion of the cover might be a dry mount of 
the same diatoms.) 
My course is to find out, in the first place, what the field of the 
objective to be measured is, by means of a stage micrometer. I 
then cut the width of the field or less, as I choose, through the 
silvering, guiding the knife against a metal straight-edge. A little 
dilute acid cleans the slit-space. In the case of the ^th measured 
in London the breadth of slit required was closely to 0-015" to 
s^an the field, an unneeded breadth for the purpose of angle-mea- 
surement. 
I will now relate my experience with the apparatus in a trial of . 
it on a ^th closely similar to the one Mr. Wenham tried his inven- 
tion on. I induced the owner of the objective, a resident of this 
city, to bring it in and witness the trial. 
Thickness of cover used ^^g- inch, which the objective at ** closed," 
just focussed through upon the objects in the balsam, or in the dry 
mounted portion, with air contact. The edges of the slit trenched 
slightly upon the field of view in the eye-piece, a " B " of medium 
aperture. 
To get the angle as Mr. Wenham got it, the uncovered portion 
of the slit was brought into view. A clear air-space between, of 
course. The angle was less than 100°, — scarcely 90°. The covered 
portion of the sht was then brought into view, cover dry. The 
angle was all my thin* stage would admit of, about 160°, but the 
aberration even with the very thick cover and contact (with brass 
setting of front lens) rendered the trial indecisive as to useful 
angle. 
But observe ; with water contact and the object-slide having been 
transferred to hottom of the thin stage, the objects were nicely 
defined even with light incident almost exactly parallel with the 
surface of the slide. 
Furthermore, with a slit one-half or one-third the breadth of 
the field, the result was just as satisfactory with water contact ; but 
with air only intervening, the angle was reduced, accordingly as the 
field was reduced, from less than 100° (small enough !) to one-half 
or one-third that angle. 
Properly used, it is evident that for taking angles a narrow slit 
of field-aperture is as good as more, and therefore a slit at the focus 
suitable for yV^^ might serve for Jth. In fact, what suits a ^^th 
ought to serve for all joowers helow. The thinnest covering glass 
will do, that being needed merely to secure the objects, as tests 
of definition, in place. The space may be filled up with glycerine 
to contact as well, ordinarily. Always, where only infinitely near 
to 180° of aperture is to be measured. Water has too low refrac- 
tion to correct in place of glass. 
♦ Not zinc " / stage; see 'M. M. J.' for August, 1874, p. 65. 
