860 SUMMARY OF CURRENT RESEARCHES RELATING TO 



other fibres. The hair or fibre is placed between two pincers a 

 (exactly at their points). One of these is movable on the base-plate c, 

 by means of the screw b, and the object can therefore be stretched, and 

 the extent of the stretching read off on the scale on the plate. As, 

 moreover, they each move on their axis, the object can be uncurled 

 in case it is twisted, and the movement registered on a scale on the 

 end of the screw d, to which an index is also attached. In order to 

 be able to measure the various diameters of the object, it is necessary 

 sometimes to turn it entirely round. For this purpose the bar e is 

 added, whose two milled heads / are pressed towards the correspond- 

 ing ones of the pincers a by means of the screw g, so that they act 

 like cog-wheels. The simple turning of the bar e by means of the 

 third milled-head h sets both the pincers in equal rotation. 



This instrument also provides means for chemical treatment. For 

 this purpose the base-plate has two spring-pieces I for the reception 

 of a small slide. These supports are raised up when the pressure of 

 the screws m is released, so that the object may lie on the slide. A 

 glass cover can be placed over it, and the object treated in the usual 

 way with alkalis, acids, &c. 



By sliding the apparatus upon the plate n (clamped to the stage of 

 the Microscope), the object can be passed across the field. 



Gundlach's Substage Refractor.* — The formula in the descrip- 

 tion of this apparatus at p. 692 was taken verbatim from the original 

 source, and in the bibliography at p. 699 we noted a further article by 

 Mr. Gundlach (with a different heading) as " apparently the same as 

 the preceding." On comparing the two articles, however, it will be 

 seen that the earlier one was somewhat hastily prepared, and that the 

 formula should stand as in the later one as follows : — 



" For the determination of the angular aperture of objectives, if not 

 less than 96° in crown glass, I propose to attach to the front surface of 

 the objective, by means of a ' homogeneous ' medium, in the usual way, 

 a small piece of crown glass, which has, besides the adhering surface, 

 two other polished plane surfaces at right angles to the former and 

 parallel to each other, with a distance between them of at least the 

 diameter of the front lens of the objective. 



Then from two distant points, lying in the plane described by the 

 optical axis of the objective and the perpendicular upon this axis and 

 the parallel plane surfaces of the glass piece, let rays of light fall 

 upon these surfaces, to pass through the glass and then through the 

 objective. 



Find, in the usual manner, by moving the lights sideways, that 

 direction of the two light rays by which the latter will just strike the 

 outer edge of the aperture of the objective. Then determine the angle 

 described by the two rays before entering the glass piece, and find 

 the true crown-glass angle of the objective by calculation after this 

 formula : — 



i being half the angle of the two rays before entering the glass piece ; 

 * Amer. Mon. Micr. Journ., iii. (1882) p. 176. 



