346 MICROMETRY. 



this method. We believe Dr. Woodward preferred this to all 

 others for the measurement of blood corpuscles. By substituting 

 the stage micrometer for the object its image is thrown upon the 

 ground glass, and then can be measured or photographed as 

 desired. Measurements should be made upon the negative and 

 not on the positive print obtained from it. 



Regarding the unit of measure to be adopted in micrometry, 

 it may be said that the micron, or the i/ioooth of a millimeter, is 

 now generally adopted in scientific work. The standard was pro- 

 posed by Harting in 1859, who called it a micro-millimeter. This 

 term was afterwards changed to micron by Listing, and the sign ^ 

 adopted as an abbreviation. The micron is equivalent to nearly 

 1/25, 000th of an inch. The determination of the magnifying 

 power of a microscope is effected in much the same way as the 

 measurement of microscopic objects. 



One of the simplest plans is that suggested by Hooke, to 

 which reference has already been made. If a stage micrometer 

 is used in place of the object a comparison can be readily made 

 between the image of any portion of the scale and the ruler placed 

 at the same distance. Thus if i/iooth of an inch on the stage 

 micrometer is equivalent to 6/ioths of an inch on the ruler, the 

 magnification is sixty diameters. 



Another simple and accurate method is to use some form of 

 the camera lucida. The microscope is arranged as for drawing, 

 the paper being placed ten inches from the eye-piece. It is im- 

 portant to note that in measuring objects by the camera lucida it 

 is immaterial what this distance is, since the object bears the same 

 ratio to the scale at all distances. A micrometer is then placed 

 upon the stage, and its graduations projected upon the paper and 

 measured. The magnifying power is found by dividing the dis- 

 tance measured on the paper by the division of the stage niicro- 

 meter used. Thus, if i/iooth of an inch of the scale equals three 

 inches on the paper, then the magnifying power is 300 diameters. 



Other methods have been suggested, but they possess no 

 advantages over those given. 



