i8 7 i.] 



Light. 



273 



tissues of the hardness of those mentioned. The section, when dried with 

 blotting paper, but still moist,* is to be mounted upon a slide on which 

 Canada balsam has been hardened, and covered with a thin glass on which 

 balsam has been similarly hardened, the moist surface prevents the balsam 

 penetrating the lacunae and canaliculi and obliterating the structure. With 

 practice, from the time of cutting the slice till the slide is ready for the cabinet, 

 about half an hour will have elapsed. 



A machine for grinding sections of hard substances for microscopic purposes 

 is described by Mr. C. Sellers, in the " Proceedings of the Academy of Natural 

 Sciences of Philadelphia." It consists of an emery or corundum wheel similar 

 to those employed by dentists, over which is placed a plate having a circular 

 aperture, through which a portion of the edge of the wheel projects ; the amount 

 of this projection is capable of being altered by means of a screw adjustment. 

 The substance to be ground is attached by a suitable cement to a glass slide 

 and moved over the aperture while the wheel is rotating until a level surface 

 is obtained. The contrivance is ingenious, but for producing perfectly plane 

 surfaces nothing can equal the lap or horizontal wheel commonly used by 

 opticians and lapidaries ; the edge of a wheel has a constant tendency to grind 

 a hollow place, and most careful shifting of the object is required to secure any 

 approximation to truth. 



Dr. Barker brings before the Royal Irish Academy an adaptation of the 

 immersion principle for the illumination of microscopic objects. The good 

 qualities of hydro-objectives or immersion lenses, with respect to improved 

 definition, increase of light, working distance, and magnifying power, are 

 already well known ; corresponding advantages are to be obtained by the use 



of illuminators constructed on a similar 

 plan. Dr. Barker has at present only 

 experimented on the principle as applied 

 to Wenham's paraboloid, which he con- 

 structs with a flat top (Fig. 17), instead 

 of the usual hemispherical cavity, and 

 introduces a film of water between it 

 and the under surface of the slide, thus 

 securing optical contact between the 

 paraboloid and the slide ; the film of fluid 

 will also act as a water joint, and allow 

 free action to the stage movements, so 

 that any part of the slide can be easily 

 examined. The oblique rays are thus 

 economised ; little dispersion takes place 

 if the object is mounted in fluid or balsam, 

 and there is sufficient brightness for all 

 Another advantage arises unexpectedly, for if the focus of the para- 

 slide of extra thinness be used, the 



Fig. 17. 



powers 



boloid be made a little higher, or if 



Fig. 18. 



oblique rays will undergo total reflection from the upper surface of the covering 

 glass and be sent down on the object so as to illuminate it by reflected light 



* Mounting a specimen damp in balsam must sooner or later give rise to a cloudy appearance 

 in the surrounding medium ; if the balsam is well hardened and used with the smallest 

 necessary amount of heat, there is but little fear of the canaliculi being filled. 



VOL. VIII. (O.S.) — VOL. I. (N.S.) 



2 N 



