1 92 



FOCUS OF OCULARS 



[CH VII 



whole microscope is obtained as described in § 176. As the initial 

 power of the objective is known, the power of the whole microscope 

 must be due to that initial power multiplied by the power of the oc- 

 ular, the ocular acting like a simple microscope to magnify the real 

 image (Fig 26). 



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 : (o ; i ::f:25omm.) 

 5:50:^:250. Whence f=25, the equivalent focus of the ocular. 



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., 1870, pp. 149-159. J. J. Woodward on the Nomenclature of Achromatic 

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

 1872, pp. 66-74. W. S. Franklin, Method of determining focal lengths of 

 microscope lenses. Physical Review, Vol. I, 1893, p. 142. See pp. 1119-1131 

 of Carpenter-Dallinger for mathematical formulae ; also Daniell, Physics for 

 medical students; Czapski, Theorie der optischen Instrumente; Dippell, 

 Nageli und Schwendener, Zimmermann. E. M. Nelson, J. R. M. S. 1898, p. 

 362, 1900, pp. 162-169. J our - Ouekett Micr. Club, vol. V. pp. 456, 462. A. E. 

 Wright, Principles of Microscopy, and in Jour. Roy. Micr. Soc. , 1904, p. 279; 

 Spitta, Microscopy; Edser, Light for Students; Conrad Beck, Cantor Lectures. 



Fig. 147. Simple Demonstration Microscopes. The upper figure has a 

 kind of stage with clips to hold the specimen. The lens may be focused up and 

 down by sliding it on the standard. For observation it is held between the 

 eye and the source of light. In the lower figure the lens is supported by a 

 handle and may be used something as a reading glass. (From Leii~' 

 Catalog. ) 





