806 APPENDICES 



III. EEFKACTIVE INDICES 



Approximate refractive indices of some substances occurring 

 and used in microscopic work, for the D (sodium) line. 



Air ... ... . 1-0 



Water . . . . . 1-334 



Alum (sat. sol.) . . . 1-358 



Absolute alcohol . . . . 1-368 



Fluor-spar . . . . . . 1-438 



Cedar- wood oil . ." . . 1*510 



Canada balsam .... 1*540 



Styrax 1*582 



Monobromide of naphthaline . 1*657 



Kealgar . . . . 2*549 



Crown glass .... 1*530 (about) 



Flint glass .... 1*540 to 1*710 



IV. THE POSITION OF A PHOTOGKAPHIC LENS IN KELATION 

 TO THE FOCUSSING-SCREEN OF THE CAMERA WHEN A 

 DEFINITE DEGREE OF ENLARGEMENT OR KEDUCTION 

 OF AN OBJECT OR PRINT is KEQUIRED 



The following short tables indicate the relative position 

 of the lens and camera-screen in photographing certain micro- 

 scopic objects. The measurements may be taken from the 

 diaphragm of the lens, it being assumed that in nearly all 

 cases a symmetrical combination will be used and not an 

 ordinary single achromatic lens. Should a single lens be 

 used, the distance may be estimated from the back surface 

 of the lens. 



Number of Times of Enlargement' 



128456 



Focus of Lens. Distance in Inches of Lens from Focussing-screen. 



3 inches. 



4 



5 



6 

 9 



