CH. VlII-\ PHOTO-MICROGRAPHY 211 



of the ground surface must be exactly at the level where the sensi- 

 tive film of the plate is in taking the picture. Focus the focusing 

 glass accurately upon the diamond scratch and fix the glass so that 

 it will remain at exactly that distance from the diamond scratch. 

 Then when an object is to be focused if the image is perfectly sharp 

 under the focusing glass its real image will be at the proper level 

 for taking the picture. 



A still more satisfactory method for the final focusing is to have 

 one of the better forms of focusing glass mounted in a board screen, 

 then one looks at the aerial image formed by the objective exactly 

 as in looking into a microscope. One must take especial pains in 

 setting the focusing glass in the board screen so that the real image 

 will be at the right level. One can do this by placing a ground 

 glass in the plate holder and putting it in position on the camera 

 and then focusing some printed letters as sharply as possible. This 

 will get the real image at exactly the right level for the plate holder 

 to be used in making the picture. Then the board screen with the 

 focusing glass is put in place of the plate holder and the focusing 

 glass moved up and down until the image is as sharp as possible. 

 The focusing glass is then fixed in position, and any object focused 

 ■with it should be equally in focus on the sensitive film for making 

 the picture. 



This method has the great advantage that there is nothing 

 between the focusing glass and the aerial image, and one can focus 

 as easily and certainly in this way as with a compound microscope. 

 For this final focusing it is better to have the diaphragm opening 

 as it is to be in taking the picture, although for getting the general 

 focus and arranging the object when the ground glass is in position 

 the full opening of the objective may be used for the greater 

 illumination. 



§ 283. Exposure. — In daylight with white embryos and a 

 dark ground 30 to 40 seconds is usually sufficient exposure. One 

 must learn this also by trial and it facilitates the obtaining of exact 

 data to make a record of every negative made, whether the negative 

 is good or bad. A table is given in § 316 to facilitate the record 

 taking. In a short time one can learn to make the correct exposure. 

 If the result is unsatisfactory, try again. The rule adhered to by 

 all first rate workers is to stick to it until the result is satisfactory. 



§ 284. Records of Embryos. — Each specimen or group of 



