6i2 FOOT AND STROBELL. [Vol. XVI. 



glass. This difference we found to be ^\ of one of the 

 twenty-five divisions marked on face of micrometer screw. ^ 



In the practical use of this method, the suggestion for 

 increasing its accuracy came through observing the variation 

 in the turn required by the micrometer screw, dependent 

 upon the operator's eyeglasses, whether reading or distance 

 glasses were worn. This suggested that there must be a 

 lens which, when once adapted to the operator's eyes, would 

 give with unfailing accuracy the plane required for the focus 

 on the ground glass. A series of spherical lenses, from — i.D. 

 to — 5.D., were tested in the following manner: 



A number of small microsomes were carefully focussed 

 through the microscope, with projection ocular IV (diaphragm 

 at o), the operator wearing ordinary distance spectacles, or in 

 a case where the sight was normal, no glasses were used. 

 The micrometer screw was then turned to raise the focus 

 the number of points found by former tests, to give the cam- 

 era focus. By this change of screw the microsomes origi- 

 nally focussed upon were of course completely lost sight 

 of. Leaving the screw at this point, the spherical (minus) 

 lenses were tested, beginning with the lowest number, placing 

 them one at a time on the projection ocular, until one was 

 found which brought the microsomes in sight and gave the 

 desired focus, exact in every detail. In making this test, if 

 the camera focus has not been obtained mechanically {i.e., the 

 difference in the two foci measured by points on micrometer 

 screw) the most accurate way of getting at it is to take a 

 series of photographs, focussing through the minus lenses. 

 Beginning with the lowest number, develop each plate as it is 

 taken, until a negative giving the desired focus demonstrates 

 which lens is needed. The proper lens, when found, can be 

 mounted in ordinary spectacle frames, or simply laid on top of 

 the projection ocular, as the operator finds most convenient. 



To insure faithful results it is advisable to make a careful 

 sketch, for reference, of a few of the most minute details, 

 in the section to be photographed. After focussing for a 



1 These figures are given merely to illustrate the method, each microscope 

 requiring a special test. 



