ZOOLOaY AND BOTANY, MICROSCOPY, ETC. 



905 



to front is effected by S^, whicli has an endless screw working in the 

 toothed wheel E on the axis C, which turns in the tube H attached 

 to the fixed plate, C being connected with the movable plate B, the 

 latter will describe an arc of a circle when Si is turned. 



Fig. 167. 



By way of apology for the latter movement not being rectilinear, 

 it is pointed out that " since the lever-arm from the turning-point at 

 C to the centre of the optic axis is very large in comparison with the 

 diameter of the field, so that the arc described is a very flat one, the 

 movement will not appear to be circular, but rectilinear." 



A finder is made by graduations at Be and N for the one move- 

 ment, and on a disk at T (with an index z) for the other. The small 

 screws s s serve to regulate the tightness or looseness of the movement 

 of the slide M. 



Fol's Compressor.* — Mr. J. A. Eyder writes us that he finds the 

 compressor of Prof. H. Fol the most convenient he has tried ; in fact, 

 his studies of the development of living fish ova could not have been 

 accomplished without it. 



It consists (fig. 168) of an octagonal base-plate 3J in. by 2^ in., with 

 a circular aperture closed by glass disk 1^ in. in diameter. A raised 

 rim round the disk gives a depth of l-6th in. for fluids. Over the 

 aperture is a sprung brass ring attached to an arm, which is moved 



* Morph. Jahrb., ii. (1876) pp. 440-4 (1 fig.). 

 Ser. 2.— Vol. III. 



3 p 



