CH.X] TEST PLATE AND APERTOMETER 273 



values, O - ,-„, I -];| then |^m.:i 111111. ::F: 250 whence F— 50 mm. That is, the 

 equivalent focus is approximately 50 millimeters. 



'{ 434. Determination of Initial or Independent Magnification of the Objec- 

 tive. — The initial magnification means simply the magnification of the real image 

 (A'B', Fig. 21) unaffected by the ocular. It may be determined experimentally 

 exactly as described in \ 433. For example, the image of the object ( -j-,, mm.) 

 measured by the ocular micrometer, at a distance of 250 mm. is \% mm., i. e., it is 

 five times magnified, hence the initial magnification of the 50 mm. objective is 

 approximately live. 



Knowing the equivalent focus of an objective, one can determine its initial 

 magnification by dividing 250 mm. by the equivalent focus in millimeters. Thus 

 the initial magnification of a 5 mm. objective is - T :"- = 50 ; of a 3 mm., -5" =83.3 ; 

 of a 2 mm., -}'i = 125, etc. 



\ 435. Determining the Equivalent Focus of an Ocular. — If one knows the 

 initial magnification of the objective (''/, 434) the approximate equivalent focus of 

 the ocular can be determined as follows : 



The field lens must not be removed in this case. The distance between the 

 position of the real image, a position indicated in the ocular by a diaphragm, and 

 the back lens of the objective should be made 250 mm., as described in \ 433, 434, 

 then bv the aid of Wollaston's camera lucida the magnification of the whole mi- 

 croscope is obtained, as described in { 160. As the initial power of the objective 

 is known, the power of the whole microscope must be due to that initial power 

 multiplied bv the power of the ocular, the ocular acting like a simple microscope 

 to magnify the real image (Fig. 21 ). 



Suppose one has a 50 mm. objective, its initial power will be approximately 5. 

 If with this objective and an ocular of unknown equivalent focus the magnification 

 of the whole microscope is 50, then the real image or initial power of the objective 

 must have been multiplied 10 fold. Now if the ocular multiplies the real image 

 10 fold it has the same multiplying power as a simple lens of 25 mm. focus, for, 

 using the same formula as before : = 5:1 = 50 : : F: 250 whence F = 25. The 

 matter as stated above is really very much more complex than this, but this gives 

 an approximation. 



For a discussion of the equivalent focus of compound lens-systems, see 

 modern works on physics ; see also C. R. Cross, on the Focal Length of Micro- 

 scopic Objectives, Franklin Institute Jour., 1S70, pp. 401-402; Monthly Micr. 

 Jour., 1S70, pp. 149-159 J. J. Woodward on the Nomenclature of Achromatic 

 Objectives, Amer. Jour. Science, 1S72, pp. 406-414 ; Monthly Micr. Jour., 1872, pp. 

 66-74. W. S. Franklin, method for determining focal lengths of microscope 

 lenses. Physical Review, Vol. I, 1S93, p. 142. See pp. 1119-1131 of Carpenter- 

 Dallinger for mathematical formulas ; also Daniell, Physics for medical students ; 

 Czapski, Theorie der optischen Instrumente ; Dippell, Nageli und Schwendener, 

 Zimmermann. E. M. Nelson, J. R. M. S. 1S9S, p. 362, 1900, pp. 162-169. J our - 

 Quekett Micr. Club, vol. V. pp. 456, 462. 



\ 436. Drawings for Photo-Engraving. — The inexpensive processes of repro- 

 ducing drawings bring within the reach of every writer upon scientific subjects 

 the possibility of presenting to the eye by diagrams and drawings the facts dis- 

 cussed in the text. Though artistic ability is necessary for perfect representation 

 of an object, neatness and care will enable anyone to make a simple illustrative draw- 

 ing, from which an exact copy can be obtained and a plate prepared for printing. 



