XVIII, 4. Foot a. Strobell: A method of focussing in photomicrography. 425 



done , after replacing the — 7 D. lens, without looking through 

 the microscope. One or more hairbreadth turns will bring the pre- 

 paration into the right plane again. Examine the preparation after 

 the first turn , and if the focns is still too low , move the screw 

 agin very slightly without looking at the preparation, and repeat 

 the experiment until the eorrect focus is secured. This mechanical 

 way of moving the screw, minimizes the chance of turning it beyond 

 the point needed. If, 011 the contrary, the higher lens ( — 9 D.) gives 

 the best focus, the micrometer screw rnust be turne d down, revers- 

 ing the process described above. The stronger lens shows a lower 

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

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

 weaker lens a lower plane. 



The simplest method of determining which lens the Operator 

 needs for a given conibination of objective, ocular and camera draw, 

 is to select a very tine detail, easily thrown out of focus, using a's 

 a test a lens of medium strength, for example, — -5 D. After waiting 

 for the focus to hold, expose the plate, but before developing, examine 

 the preparation, again replacing the — 5 D. lens to see that the 

 focus held during exposure. If it has held, develop the plate. If 

 the negative shows too high a focus (i. e., too high a plane) you 

 need a lens of lower number than the No. 5, and conversely, if the 

 focus is too low, a higher power lens is needed. Without touehing 

 the micrometer screw, look at your preparation through the different 

 lenses , until you find one which gives the focus s h w n in the 

 negative, and that is the lens you need for this conibination of 

 objective, ocular and bellows draw. 



Explanation of Plate III. 



Nine vignetted photographs of one section of an egg of Allolobophora 

 foetida, showing lower pole of first maturation spindle and two and a half 

 of the eleven chromosomes. No. 1 — 6, magnification 660: Zeiss 2 min obj. 

 projection ocular 4, diaphraghm at 0, bellows draw 21 1 /., inches (54"5 cm) 

 from stage of microscope to plate holder of camera. 



These figures (1 — 6) showing six consecutive planes of one section 

 2 x / 2 fj thick, were taken by focussing through six minus spherical lenses 

 from — 7 D. to - - 12 D. We have reproduced these photographs to show 

 the mechanical exactness of this method of focussing. No. 1 was focussed 

 through a — 7 D. lens. This number giving the Operator the eorrect 

 focus for the above combination of objective, ocular and bellows draw. 

 A sharp focus on the middle chromosome was chosen. 



