SOME SPECIAL PEOCESSES 273 



To reproduce an object in its natural size, the distance 

 of the ground-glass screen of the camera from the lens itself 

 must be double the focal length of the lens (see Appendix IV). 

 This may be determined with sufficient accuracy by taking 

 the focal length of the lens which is usually marked on the 

 mount measuring the distance from the iris-diaphragm of the 

 lens to the ground-glass focussing-screen, and making this 

 double the focal length. Obviously, a camera will be required 

 that has sufficient extension, but most half-plate field cameras 

 are long enough. The object must then be placed at 

 approximately the same distance from the lens-diaphragm. 

 In cases where the image is required to be larger than the 

 original, unless the lens is of the symmetrical type, it 

 should be reversed in the camera-front ; that is, the back 

 combination of the lens should be placed towards the object, 

 and the image projected with the front of the lens facing 

 the ground-glass screen. In this case no definite instructions 

 can be given as to distance of object or extension of camera, and 

 these must be determined by experiment to suit the particular 

 case. Where symmetrical lenses are used such as a Zeiss 

 double Prolar then moderate magnification, up to ten or 

 fifteen diameters, may be obtained with the lens in its normal 

 position in the camera-front. 



The appliances already described for dealing with culture- 

 tubes and plates may be also used for pathological or similar 

 specimens. In this case, the object may be supported in the 

 position C, and illumination effected as with culture-tubes in 

 which the growth is on the surface. It is sometimes difficult, 

 however, with such objects to support them conveniently in a 

 vertical position, and it is therefore an advantage to have all 

 the appliances described fastened to the baseboard, so that the 

 camera may be supported in the vertical position. The 

 object may then be laid on a glass plate, and illumination 

 effected from above or below as required. With pathological 

 specimens a source of trouble is that they are often in a wet 

 condition, and therefore some parts have a glistening appear- 

 ance. In such cases they should, if possible, be surface-dried by 

 immersing them in alcohol for a short period. Where this 

 cannot be done, it is often permissible to dust lightly over 

 their surface some fine granular powder, such as powdered 



