162 PHOTOMICROGRAPHY 



of a preparation, or one colour over the other in double - 

 stained sections. Colour screens are useful for all these 

 purposes. 



The degree of contrast in the photomicrograph depends 

 on the colour of the object itself, and on the sensitiveness 

 of the plate to the various colours of the spectrum, and 

 has little or nothing to do with the performance of the 

 lens. Fortunately the means adopted to overcome any 

 lack of contrast in many cases obviate the results of 

 aberrations, and improve definition and resolution at the 

 same time. 



Increase of Contrast. The greatest contrast in a photo- 

 graph is obtained when part of the subject is black and 

 part white, so that any details or parts of a microscopic 

 object illuminated by transmitted light will stand out 

 in the greatest possible contrast to the surrounding parts 

 if they can be made to transmit no light and appear 

 black. How such a state of things may be brought 

 about will perhaps be best understood if the object is 

 considered for the moment as a light filter passing a 

 definite band of colour depending on the stain used in 

 its preparation. For instance, suppose a section is 

 stained with picric acid and has an absorption corre- 

 sponding to that of the d}~e which is indicated on page 159 

 as extending from the blue end to 4600. Then suppose 

 an ammoniacal copper sulphate filter is placed in the 

 course of the beam of light before it reaches the prepara- 

 tion. This solution has an absorption which exactly 

 covers the transmission of the picric acid, and the 

 effect will be that the section and screen together will 

 transmit no light and appear black, and the condition for 

 the greatest contrast between object and background in 

 the photomicrograph is thus attained. 



Two screens, such as ammoniacal copper sulphate and 

 picric acid solutions, whose transmissions together make 

 up a complete spectrum band, are known as " comple- 



