at the Surface of Highly- absorbing Medic 



351 



maximum for red light. If, however, we look at the film 

 through the glass on which it is deposited, the intensity first 

 diminishes with increase of thickness, reaching a minimum 

 in the bluish neutral colour of II. ; it then rises to a maximum 

 in the bright orange of III. 



Green Light. — Fuchsine is very opaque to green light. 

 No maximum or minimum can be seen if we look directly at 

 the film. Seen from the back of the glass, there is one 

 minimum which occurs in the dark red of II., that is at a 

 somewhat greater thickness than the first minimum for yellow 



li 8 ht 



Blue Light. — In blue light, for which fuchsine is consider- 

 ably more transparent than for green light, there is a marked 

 difference in the position of the maxima and minima as seen 

 directly and through the glass. Looking directly, there is a 

 maximum near III. and a minimum between IV. and V. nearer 

 to IV. than to V. 



The following is a fable showing roughly the result of these 

 observations. The Roman numerals refer to the same thick- 

 nesses as in Table A : — 



Table B. 



Thickness. 



Loc 



I. 



king at film diret 



jtly. 



Lookin 



II. 



g at the film from back 

 of the glass. 



Incident Light. 



Incident Light. 





Eed. Yellow. 



Green. 



Blue. 



Eed. 



Yellow. 



& reen . 



Blue. 



Zero 



Ill 



Max. 



Max. 



Max. 

 Min. 



Max. 

 Min. 

 Max. 



Min. 



Max. 

 Min. 

 Max. 



Min. 



Min. 

 Max. 



IV 



Min. J Min. 

 Max. Max. 



V 















§ 3. Table A and Table B. 



The appearance of the film (Table A) is different according 

 as the film is seen directly or through the glass. This differ- 

 ence is partly due to the fact that the selective reflexion at the 

 surface of a highly absorbing substance depends upon the 

 refrangibility of the neighbouring transparent medium *. 

 The surface-colour of a thick layer of fuchsine seen through 

 the glass on which it is deposited is much more blue in tint 

 * E. Wiedemann, Pogg. Ann. cli. 



