ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 127 



particularly crystals whicli will not polarize, diatoms, infusoria, 

 palates of molluscs, &c., can not only be seen to greater advantage, 

 but their parts can be more easily studied. As its cost is merely 

 nominal, it can be applied to every instrument, large or small, and 

 when its merits and its utility by practice are known, I am confident 

 that it will be considered a valuable accessory to the Microscope." 



Gundlach's Substag^e Refractor. — E. Gundlach publishes direc- 

 tions (with a table) for using this apparatus for the determination of 

 the aperture of objectives from 1 • 13 N.A. to 1-51 N.A. (97° to 180° 

 in crown glass). 



The refractor (described Vol. II. (1882) pp. 692 and 860) consists 

 of a small cube of glass, having one blackened and several polished 

 surfaces. 



To use it, screw on the objective, and in place of th.e eye-piece, 

 put a diaphragm having an opening about l-4th in. in diameter. 

 Then to the front surface of the objective, with a very small drop of 

 Canada balsam, make the refractor adhere by that surface which is 

 opposite the blackened one, in such a position that the two polished 

 side surfaces will stand vertical when the body is brought into a 

 horizontal position. Let the balsam harden a little ; place the body 

 in a horizontal position, and turn the mirror to one side to get it out 

 of the way. 



Then place two lights — ^flat-wicked oil lamps are best — at some 

 distance from the Microscope, say six or eight feet, one on each side 

 of the optical axis, and at first pretty near this axis. By looking 

 through the diaphragm at the eye-piece end, towards the objective, 

 the two lights will appear there as two small light-spots, presuming 

 the angle of the objective to be large enough. If they do not appear, 

 and also will not, or at least one of them, when the ligbts are brought 

 very near together, then the angle of the objective is smaller than 96° 

 or 97° in crown glass, according to the index of refraction of the 

 crown glass used in th.e refractor ; and the angle cannot be determined 

 with it. If tbey appear, move both lights slowly away from the axis 

 and find carefully the place for each where its image in the objective 

 will just disappear. Determine th.e angle described by the light-rays 

 entering the refractor at each side from each lamp, either by measuring 

 directly, or by measuring the distance of the lamps from each other 

 and from the refractor, reducing the distance of the lamps by the 

 thickness of the glass cube, and finding from these three measure- 

 ments, as the sides of a triangle, the desired angle by calculation. 



Compare the angle thus found with those given in column A of 

 the table, and find the one which is nearest to the determined angle. 

 The corresponding angle of column B is the crown-glass angle of the 

 objective, and the corresponding number of column C is its numerical 

 aperture. The balsam may be removed easily and safely with a 

 little benzine. 



ToUes' Frontal-prism Illuminator. — Fig. 22 shows an arrange- 

 ment devised by Mr. R. B. ToUes to be applied to the front of a 

 1-in. objective for illuminating opaque objects. 



