THE MICROSCOPE. 29 
will be obvious that those rays which are transmitted 
-by the parabola impinge normally to the surface of the hemis- 
pherical lens and hence are not refracted; that is, they traverse 
the same path in the lens that they had upon the parabola. The 
effect then, is that of the wide angled immersion condenser with 
the central rays stopped out. Although this may be very desirable 
for some objects it is not generally so, and it becomes necessary to 
limit the direction from which the light comes. This may be very 
easily accomplished by the use of a cardboard diaphragm. ‘This 
may be made by cutting a circle of blackened cardboard, the 
diameter of the inside of the mounting of the parabola, so that 
when crowded home against the glass surface the circle will be held 
friction tight. By cutting small holes in this card the light may be 
regulated; and it should be kept weil in mind that when the holes 
are cut in the outer edge of the card, the light, although oblique, 
will be more nearly central than when admitted to the reflecting 
surface through a hole nearer the center; but should the hole be too 
near the center of the card the light will not be transmitted at all, 
owing to the fact that it will strike the top of the concavity of the 
parabola. A good guide to go by is acircle upon the card whose 
diameter is the same as that of the top of the concavity. The most 
oblique light may then be obtained by cutting the holes near this line. 
Holes may be cut at various angles to each other, to effect the 
resolution of the various sets of lines by which some objects are 
marked. ‘ 
a 
a ee 
a 
In the figure the position of the lens and parabola as regards . 
the slide is shown. S. represents the slide; L. the lens; P. the 
