240 E. M. NELSON ON A NEW METHOD OF MEASURING THE 



are large and heavy, are packed away ; it would take some 

 hours to unpack them, clear out a room for their reception, 

 bring them in and set them up, so I have to be content with 

 some other means of measuring magnifying powers. 



The second method, viz. that of employing a camera lucida, 

 also appears to be very simple, and so it is when a Powell No. 1 

 stand is used, which has its optic axis ten inches from the table, 

 when inclined horizontally ; a Beale's neutral tint fitting on the 

 " capped" eyepieces answers perfectly some attention, however, 

 is necessary to regulate the illumination, both in the tube and on 

 the rule, otherwise the coincidences of the lines cannot be 

 observed. But suppose a continental eyepiece is used, what 

 then ? The Beale camera will not fit, and all the simplicity of 

 one's arrangements and apparatus fails. If the microscope is 

 not a Powell's No. 1, then it must be placed upon a box, and the 

 distance of the rule adjusted by means of other boxes, books, 

 etc. With a Continental microscope matters are no better. One 

 has the simplicity of the Abbe camera, with its cleverly planned 

 device for regulating the illumination of the stage micrometer 

 and of the rule, but suppose the eyepiece is of the positive com- 

 pensating type, what is to be done ? The camera will not fit 

 and cannot be used. There are other cameras, both of the right- 

 angled and of the oblique type ; some eyepieces they fit and 

 others they do not. These difficulties are not imaginary, for 

 I have experienced all of them at one time or another. The 

 apparatus I now use is the old-fashioned Wollaston's camera, 

 mounted on a table screw clamp. This can be used with every 

 kind of eyepiece ; it is, however, troublesome to work with, and 

 it requires some practice to obtain a coincidence of the scales 

 how anybody can execute a drawing with such an apparatus is, 

 to me, quite incomprehensible ! 



I have devised an entirely new method by which all these 

 worrying little troubles may be avoided. First, it is necessary 

 to determine the " constant " of the eyepiece with a given tube 

 length. This is easily done, and when done it should be recorded, 

 or better still engraved on the eyepiece tube. To find the 

 " constant " of an eyepiece with a given tube length, first 

 determine the combined magnifying power of that eyepiece on 

 the given tube length with any objective, say one of medium 

 power, sudh as a |-in. or J-in. or |%-in. focus. Secondly, measure 



