No. 3-] EGG OF ALLOLOBOPHORA FCETIDA. 537 



on slightly different planes, noting the effect raising or lower- 

 ing the micrometer screw has on each microsome. Thus the 

 slightest slip of the focus can be detected at a glance, and 

 the relative sharpness of the two microsomes will indicate in 

 which direction it has changed. A further aid in settling any 

 doubt as to whether the focus has slipped during the inter- 

 val of rest is to glance at the preparation through a stronger 

 and weaker focusing lens — 6'.^., if you have focused with a 

 — 7.D., and the preparation looks sharper through a - 8.D., 

 the focus needs to be slightly lowered. If, on the contrary, 

 the image appears sharper through a -6.D., the focus needs 

 to be slightly raised.^ The stronger lens shows a lower plane, 

 the weaker lens a higher plane. These relations are reversed, 

 however, in the negative, i.e., the stronger lens shows a higher 

 plane, and the weaker lens a lower plane. 



The results of our experiments with different combinations 

 of lenses and camera draw may be of some practical value, 

 although some of the experiments have not been repeated 

 often enough to warrant our offering them as infallible 

 guides. 



For the photos taken at a magnification of about 660 diam- 

 eters, we use a Zeiss 2 mm. immer. 140 aperture, projection 

 ocular 4, diaphragm at o and 21}^ inch camera draw (measuring 

 from stage of microscope to plate-holder). For a magnifica- 

 tion of about 1000 we extend the draw to 29^ inches. For 

 the photos taken at a magnification of 286 diameters, Zeiss 

 4 mm. apo., the collar at the lowest point and a thin cover- 

 glass, the camera draw and projection ocular being the same 

 as for the 660 magnification. With a 2 mm. immer. projec- 

 tion ocular 4, the same focusing lens has given a satisfactory 

 focus, whether the diaphragm of the projection ocular 4 is at 

 o or 10. The magnification of the latter is, however, so much 

 less, that it is much more satisfactory to use a 4 mm. dry and 

 the 29^ inch draw ( x about 450), for the 4 mm. dry has greater 

 depth. 



1 The focus should be corrected by hair-breadth turns of the micrometer screw 

 without looking through the eye-piece, for when watching the preparation it is 

 difficult to avoid turning the screw beyond the point required. 



