ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



679 



confuse each other in the natural endeavour to blend them. This 

 requires a mental effort to exclude the impression upon the retina of 

 one eye and regard that upon the other only. Again, when we close 

 one eye by contraction of the orbicular muscle, or by pressure, as by 

 the hand, we cause contraction of the accommodating muscle also, and 

 of the other eye as well. 



To facilitate the training of both eyes the following eye-protector 

 is proposed. It consists of a small, opaque disk near the eye, sup- 

 ported by a wire extending from its outer edge downward, to a point 

 on the tube low enough to be out of the way of the nose, then bent 

 upward, parallel to the tube, but not touching it, and attached to a 

 ring near the top. Dr. Hall's is made of a piece of brass wire, 

 No. 18, about 45 cm. long ; a loop at one end, 4 cm. in diameter, 

 covered with a piece of black paper folded over and gummed down, 

 forms the disk. At the other end is a ring to fit the draw-tube, and 

 then the intermediate wire bent. It is attached below the flange, 

 on the draw-tube, where there is no lacquer to be scratched, but 

 if it should be thought desirable to attach it above the flange, 

 then the ring ought to be covered with chamois, so as not to wear the 

 polish. 



The advantages of this form are, the small size of the disk and its 

 support, interfering with the working of the instrument and view of 

 the stage as little as possible. The support is not in the way of the 

 nose ; it is elastic, not uncomfortable when touched by the nose, and 

 striking it does not displace the stand ; it can be rotated about the 

 tube and used with either eye alternately ; it can be easily adjusted to 

 the eye-distance of any worker ; and, lastly, it is of so simple a con- 

 struction that any one can make it for himself at a very small cost. 



Cramer's Camera Lucida* (also Hofmann's and Oberhauser's). 

 — Dr. C. Cramer can only concur to a small extent in the warm 

 praise which Dr. H. von Heurck has bestowed upon Hofmann's 

 camera lucida.f Besides the advantage of having the paper lie 



* Bot. Centralbl., vii. (1S81) pp. 385-91 (2 figs.). 



f Hofmann's camera lucida was described and figured in this Journal, ii. 



Fig. 122 



(1879) p. 21. We, however, add here a diagram of it (Fi>. 122), S being the 

 silvered mirror over the microscope-tube, s 2 the smaller silvered mirror which 



