Angular Aperture Defined. 53 



the working distance and the available diameter of the lens, 

 and calculate the angle. The method will be described in de- 

 tail shortly. 



First let us examine the advantages and disadvantages. The 

 disadvantages lie merely in the necessity of having a means to 

 measure working distance. 



The advantages are that an angle thus obtained is clearly 

 defined. The figures cannot be misconstrued, and scientific 

 men need not hesitate to use them for fear of being misrepre- 

 sented. 



Without taking up your time with a greater enlargement 

 upon the advantages that are to be secured in this way, I wish 

 to base my argument in favor of its adoption upon the grounds 

 first stated, viz : that we need a definition which shall admit 

 of no misapprehension, and will summarize after describing 

 the practical operation of measuring this angle, 



TO MEASURE FOCAL OR WORKING DISTANCE IN AIR, 



The best and simplest method is by means of a graduation 

 upon the body of the microscope, and upon the fine adjust- 

 ment screw head. When the face of the lens is flush with the 

 mounting, simply bring it in contact with the surface of a slide, 

 and measure the distance it has to be raised to bring particles 

 of dust which may be upon the slide into view. 



When the lens is not flush with the mounting another plan 

 is necessary. 



Place the object glass on the sub-stage as for use as a con- 

 denser, and arrange it so that it forms an image of distant ob- 

 jects at its focus. Then with another objective, say a ^-inch, 

 focus upon the face of the former, and measure the distance 

 through which the latter must be raised, in order to get a dis- 

 tinct view of the image. This will give the focal length in 

 air, 



TO MEASURE THE FOCAL DISTANCE IN FLUID, OR BALSAM. 



When the nose of the objective can be placed in the fluid 

 without injury, the slide may be marked with a diamond and 

 a little finely levigated graphite rubbed into the mark to 

 make it visible. Then interpose the fluid as usual, and meas- 

 ure as before. 



In the case of balsam, if it is not advisable to soil the ob- 

 jective with this medium, we can proceed as Mr. Wenham 



