CH. VI II .] PHOTO-MICROGRAPHY. 191 



screen. They should be carefully tested to see if there is coincidence in position 

 of the focusing screen and the sensitive film as described in {\ 324.) 



\ 330. Apparatus for Lighting. — For low power work (35 mm. and longer 

 focus) and for Urge objects some form of bull's eye condenser is desirable although 

 fairly good work may be done with diffused light or lamp-light reflected by a mir- 

 ror. If a bull's eye is used it should be as nearly achromatic as possible. The 

 engraving glass shown in Fig. 155 answers very well for large objects. For smaller 

 objects a Steinheil lens combination gives a more brilliant light and one also more 

 nearly achromatic. For high power work all are agreed that nothing will take the 

 place of an achromatic condenser. This may be simply an achromatic condenser, 

 but preferably it should be an apochromatic condenser. Whatever the form of the 

 condenser it should possess diaphragms so that the aperture of the condenser may 

 be varied depending upon the aperture of the objective. For a long time, objec- 

 tives have been used as achromatic condensers, and they are very satisfactory, 

 although less convenient than a special condenser whose aperture is great enough 

 for the highest powers and capable of being reduced by means of diaphragms to 

 the capacity of the lower objectives. It should also be capable of accurate center- 

 ing. 



{! 331. Light Filters or Color Screens. — These are solutions or suitably stained 

 collodion or gelatin films placed between the source of illumination and the ob- 

 ject. It does not make much difference where the color screen is placed provided 

 no light reaches the object which has not passed through the filter. The purpose 

 of the color screen or filter is to take out the excessive number of blue and violet 

 rays so that the more slowly acting red, yellow and green may have time to pro- 

 duce the appropriate chemical changes in the sensitive plate. This action of the 

 longer waves (see under spectroscope \ 179-19S), is greatly aided by the isochro- 

 matic or orthochromatic plates which are especially treated so that they will be 

 sensitive to the longer waves as well as to the shorter, blue and violet waves. This 

 is why it is so necessary in manipulating the plates to avoid exposing them to 

 any light whatsoever in putting them into the plate holder, developing, etc. 



The color screen used by Dr. Learning in preparing the negatives for the plates 

 in Wilson's and Starr's atlases was made by staining a lantern slide plate from 

 which all the silver salts had been removed, with an alcoholic solution of tropaeo- 

 lin and then after drying, Canada balsam and a coverglass were applied. Others 

 have recommended collodion stained with aurantia. The purpose of these is to 

 filter out the greater number of the blue and violet rays and then increase the time 

 of exposure from 2 to 5 times, depending on the thickness of the color screen. 

 Color screens and color cells are furnished by various makers. None of them 

 answers for all preparations and for some preparations they are unnecessary. 



(J 332. Objects Suitable for Photo-micrographs — While almost any large object 

 may be photographed well with the ordinary camera and photographic objective, 

 only a small part of the objects mounted for microscopic study can be photo-mi- 

 crographed satisfactorily. Many objects that give beautiful and satisfactory images 

 when looking into the microscope and constantly focusing with the fine adjust- 

 ment, appear almost without detail on the screen of the photo micrographic cam- 

 era and in the photo-micrograph. 



[f one examines a series of photo-micrographs the chances are that the greater 

 number will be of diatoms, plant sections or preparations of insects. That is, they 

 are of objects having sharp details and definite outlines, so that contrast and defi- 



