ZOOLOGY AND BOTANY, MICKOSCOPY. ETC. 



943 



homogeneous immersion by Mr. Basset,* but finds, on examination of 

 various solutions of tbe substance by the refractometer, that it is not 

 one for strictly homogeneous immersion, as it does not give as high 

 a refractive index as oil of cedar- wood, even in its most concentrated 

 form. Liquefied by the smallest possible quantity of glycerine into 

 a very thick pap, it does not give a higher refractive index than 

 1 • 510 (at a temperature of 15° C), which may therefore be taken 

 very approximately as the true index of the pure chloral. Every 

 practicable solution (even 10 or 20 parts to one of glycerine) must 

 therefore have a lower refractive index than 1 • 510 and thus leave 

 a slight defect of refraction, in comparison with ordinary crown 

 (1*520), which is possessed by cedar-oil. 



Beck's Glass Friction-stage. — This (Figs. 219 and 220) is a 

 simple form of friction-stage for application to Student's Microscopes. 

 The base-plate is circular and of brass with a deep rim to give 



Fig. 219. 



Fig. 220. 



rigidity. In addition to the usual central opening there are four 

 other circular openings of about ^ inch in diameter, one in each 

 quadrant. A polished glass plate is imbedded on the surface of 

 the base-plate, having a central opening and two circular openings 

 corresponding to those in one diameter of the base-plate. An upper 

 brass plate, on which the object-slide is placed, has a piece of velvet 

 cemented to its under surface, and it is held in contact with the glass 

 by a strong wire ring beneath. Fig. 220 shows the under surface of 

 the stage, and it will be seen that the upper brass plate (darkly shaded) 

 is attached to the ring by milled-head screws passing through the 

 right and left openings in the stage ; the other two quadrants of the 

 ring are provided with small ivory knobs which make friction-contact 

 with the under surface of the glass plate (shown slightly shaded), 

 through the other two openings in the base-plate. About ^ of an 

 inch of motion in all directions is thus allowed for the upj)er plate, 

 and the movement is extremely smooth — the friction above is between 



* See this Journal, ante, p. 123. 



