454 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



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 he 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 pushed 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 

 well 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 

 centre ; but should the hole be too near the centre 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 a circle upon the card whose diameter is the same as that of the 

 top of the concavity. The most of the 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. 



The author adds : " The chief objection to this method of illu- 

 mination is, that central light cannot be obtained ; but this, of itself, 

 is of no particular account, as the parabola 

 may be removed from the substage when it 

 is desired. As to the performance of this 

 arrangement, I can speak in the highest 

 terms ; the resolution of the diatoms of 

 MoUer's balsamed plate being easily ac- 

 complished ; and when the full operation of 

 the parabola was used, the dots of No. 18 

 showed better than I have ever seen them 

 by any other method of illumination." 



Paraboloid for Rotating Illumination in 

 Azimuth. — We have a paraboloid with an 

 arrangement shown in section in fig. 63. 

 The bottom of the fitting is closed by a 

 brass box in which is a rhomboidal prism, 

 the lower face of which is over an oblong 

 slot in the centre of the lower plate of the 

 box, while the upper face is towards the side 

 of the upper plate, and just beneath the 

 outer zone of the paraboloid. Over the 

 upper face is a tube 1^ in. high (the hori- 

 zontal section of which is shown in fig. 64). 

 Axial rays are, by means of the prism, made to fall on a part of the 

 outer zone of the paraboloid, and by rotating the box can be brought 

 into any azimuth of the latter. 



Fig. 63. 



Fig. 64. 



