194 PHOTO-MICROGRAPHY. \CH. VIII. 



with a hole of proper size is good. Then with a glass chimney on the 

 lamp, place the bull's eye between the lamp and object (Fig. 157 ). Put 

 a piece of white paper over the object and mutually arrange lamp and 

 bull's eye till a sharp image of the flame may be seen on the paper 

 covering the object (Fig. 157)- Remove the paper from the object 

 and proceed to focus. This is accomplished by sliding the whole 

 camera toward or away from the object, see also § 339. One must also 

 shorten or lengthen the bellows to get the picture of the proper size. 



In case the whole specimen is not illuminated the lamp must be 

 turned so that the broad side of the flame is toward the object. The 

 mutual arrangement of lamp and bull's eye must also be such that the 

 brightest part of the flame illuminates the object. If the bull's eye 

 is moved a little toward the lamp, the flame will be sufficiently broad- 

 ened. It must be remembered, however, that the best and most 

 intense illumination is obtained when the object appears in the image 

 of the flame. When the object is evenly illuminated and the focus is 

 made as perfect as possible, a small diaphragm is used in the objective 

 (i. e.,f $2 or 64) and the plate holder with the sensitive plate is put in 

 place of the focusing screen. Some kind of cover is put over the ob- 

 jective, the slide of the plate holder is withdrawn and then the 

 objective is uncovered. With instantaneous, orthochromatic or 

 isochromatic plates the exposure in most cases need not be over 30 to 

 90 seconds, depending on the object and the diaphragm. 



It sometimes occurs that the whole object cannot be satisfactorily 

 lighted. In that case one may use diffused day-light as follows : 

 Elevate the camera so that the object is against the clear sky for a 

 back ground. If any of the earth should form part of the back ground 

 that part of the object would not be sufficiently illuminated. It is best 

 not to use any mirror. The light from the sky will evenly and com- 

 pletely illuminate even the largest object. 



Instead of elevating the camera one might use a large reflector 

 covered with very white cloth or paper and set at an angle of 45 de- 

 grees. This reflector would then serve for back ground equally with 

 the clear sky. 



§ 336. Focusing Screen for Photo-Micrography. One cannot 

 expect a picture sharper than the image seen on the focusing screen. 

 Hence the greatest care must be taken in focusing. The general 

 focusing may be done with the unaided eye and on the ground glass, 

 but for the final focusing a clear screen and a focusing glass must be 

 used (Fig. 150). 



