ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 133 



are separated. The spring, by drawing the outer cylinder back again, 

 keeps the objective firmly in place. So far, the arrangement is 

 similar to the Nachet-Thnry form. 



To draw down the outer cylinder against the strong spring, in 

 order to release the objective, requires some 

 force, and if it is allowed to slip, the fingers are Fig. 19. 



apt to be nipped, apart from the injury to tlie 

 fine adjustment, while if the spring is weak and 

 so easily extended, the objective is only loosely 

 held. To avoid these difficulties the upper 

 margin of the outer cylinder has two notches cut 

 in it, one of which is shown in fig. 19 (the other 

 being opposite to it), whilst the inner cylinder 

 has two pins with projecting heads. When a 

 notch is opposite a pin, the outer cylinder is 

 close home, but on rotating it, so that the pins 

 do not fall in the notches, as shown in the fig., 

 the outer cylinder is forced down. 



In order to release the objective, therefore, 

 no force is required ; all that is necessary being 

 a slight rotation of the outer cylinder, so as to 

 take the pins out of the notches. To ease the 

 rotation, the pins have each a loose collar, which revolves as the outer 

 cylinder is turned. 



Hartnack's Fluid for Homogeneous Immersion.* — Dr. E. Hart- 

 nack supplies, in place of cedar-oil, vaseline oils — the white oil for 

 axial illumination and the yellow oil for the oblique. 



Rotary Object-carrier, j — Mr. J. M. Flint describes a device for 

 exhibiting a series of mounted objects, without a change of slides. 

 As described it is arranged for Foraminifera, viewed as opaque 

 objects, vsrith a low power. They are mounted on small brass discs 

 furnished with a stem, by means of which they may be carried in 

 a " Beck's disc-holder " when it is desired to make a thorough study 

 of the specimens. Ordinarily these discs are inserted in thin wooden 

 slides of regulation size and kept in boxes, until the series is com- 

 plete. In order to protect the specimens from dust or injury, and at 

 the same time maintain their accessibility, movable covers are con- 

 structed as follows : — A score or more of curtain rings, not flattened, 

 are slipped upon a squared rod of wood, and brushed over freely 

 with thick shellac. On the following day, before the shellac has be- 

 come hard, the rings are slightly separated in pairs. When the pairs 

 are firmly united, a thin glass cover is secured to the upper surface of 

 each pair, and thus a little box cover is formed, deep enough to 

 inclose disc and specimen. Now, by driving two small gimp-tacks 

 into the wooden slide, at the proper distance apart, and deep enough 

 so that the heads of the tacks will just enter the groove between the 



* Jouru. de Microgr., ix. (188.5) p. 367. 



t Amer. Mon. Micr. Jouvu., vi. (1885) pp. 204-5. 



