A PPENDIX. ] PRE PAR A TION OF DIA GRAMS. 2 1 5 



as described in \ 357. For example, the image of the object ( T 2 jinm.) 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 approxi- 

 mately five. 



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

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

 initial magnification of a 5 mm. objective is *-&£ = 50 ; of a 3 mm., £|° =83.3 ; of 

 a 2 mm., i f a = 125, etc. 



\ 359 Determining the Equivalent Focus of an Ocular.— If one knows the ini- 

 tial magnification of the objective (§ 358) 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 posi- 

 tion 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 \ 357, 358, then 

 by the aid of Wollaston's camera lucida the magnification of the whole microscope 

 is obtained, as described in \ 149. 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 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 magnifica- 

 tion of the whole microscope is 50, th,en the real image or initial power of the ob- 

 jective 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 = 5:i = 5o::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 Microscopic Ob- 

 jectives, Franklin lust. Jour., 1870, pp. 401-402; Monthly Micr. Jour., 1870, pp. 

 149-159. J. J. Woodward, on the Nomenclature of Achromatic Objectives, Amer. 

 Jour. Science, 1872, pp. 406-4^; Monthly Micr. Jour., 1872, pp. 66-74. W. S. 

 Franklin, method for determining focal lengths of microscope lenses. Physical 

 Review, Vol. I. 1893, p. 142. See pp. 1037 to 1049 of Carpenter-Dallinger for 

 mathematical formulse ; also Daniell, Physics for medical students; Czapski, 

 Theorie der optischen Iustrumente ; Dippell, Nageli und Schwendener, Zimmer- 

 mann. 



PREPARATION OF DIAGRAMS. 



$ 360. For class room work many diagrams are needed. Those which are pur- 

 chased soon get out of date, but from their cost one does not feel like throwing 

 them away. It is a fact, however, that so much of one's education conies through 

 the eye that it is not safe to have an incorrect diagram for students to study. The 

 visual impression is liable to outlive the verbal correction. To avoid incorrect di- 

 agrams or those that no longer represent the present state of knowledge one may 

 use blackboard diagrams. By means of the flexible, or roll blackboards, one may 

 make the diagrams anywhere and hang them in the lecture-room. This also is of 

 advantage where several must use the same lecture-room. 



For permanent diagrams which shall be as easy to make as the ordinary black- 



