108 THE VICTORIAN NATURALIST. 



that end can be gained by means of a general sharpness all over. 

 With the generality of subjects, however, it is best to focus some 

 characteristic part perfectly, making it well defined. If the photo- 

 graph is to be of a spider's claw the comb is perhaps the most 

 necessary part to be perfectly focussed, or if a blowfly's proboscis, 

 one would do well to give special prominence to the hairs which 

 are on the lining membrane. 



However, we will now assume that position and focus of the 

 object has been decided upon. After removing the ground glass, 

 the dark slide which contains the photographic plate upon which 

 the image is to be formed should be placed in position. Inter- 

 pose a piece of dark substance between the light and the con- 

 denser, withdraw the shutter to uncover the plate, and expose. 



As regards the duration of exposure, this is so dependent upon 

 different classes of subjects, varying qualities of illumination and 

 degrees of magnification, that it is one of the most difficult 

 matters that the photo-micrographer has to decide. In order to 

 take full advantage of the experience gained by errors and 

 successes in past attempts, it is necessary that some kind of 

 tabulated record should be kept. The following is a suggested 

 list of columns for such a record : — (i) No., (2) subject, (3) how 

 stained, (4) objective employed, (5) eye-piece used, (6) length of 

 extension of camera, (7) light used, (8) means for focussing light 

 on object, (9) exposure given, (10) developer used, (11) general 

 remarks as to result. 



Of course the staining of the specimen is an important con- 

 sideration. The colours at the violet end of the spectrum are 

 the most actinic, while those at the red end are capable of 

 producing hardly any visible chemical effect ; consequently, an 

 object which has been stained a deep red will require a very 

 prolonged exposure, and, even at that, is generally incapable 

 of yielding other than a flat, uninteresting photograph, lacking 

 in contrasts and brilliancy. Also, if an object is stained in 

 parts blue or violet and in other parts yellow, the former will 

 appear light in the resulting photograph, while the yellow rays 

 will appear dark, thus reversing their natural intensity of shade. 

 To in some measure rectify this there is what is known as an 

 Orthochromatic plate supplied by some makers, which has the 

 advantage of rendering these colours in their true gradation of 

 light and shade. These plates are effectual in receiving the 

 impressions from yellow rays, but in order to reduce the 

 intensity of blue and violet rays it is necessary to interpose 

 a piece of yellow glass between the source of light and the 

 condenser. 



The processes for developing the latent image in the 

 sensitive plate, and printing and preparing the finished photo- 

 graph, also the subject of remedying defects in negatives, are, 



