SCREENS PLATES EXPOSUEE 225 



assume that a 4-mm. objective, with an ocular magnifying 

 four diameters, is being employed, and that the diameter 

 of the Eamsden disc of the ocular, obtained by direct 

 measurement, is 1 mm. This, multiplied by the power of the 

 ocular (4), gives 4 mm. as the diameter of the objective 

 beam ; half of this, 2 mm., is the semi-diameter of the beam, 

 which, divided by 4, the focal length of the objective, gives 

 0-50 as the N.A. in use at the time. It is, of course, neces- 

 sary in this case again to make some trial exposures, and 

 to determine with a given illuminant what is the correct 

 exposure for a colourless object ; from one or more trials a 

 complete table may then be worked out which would give 

 the exposure under varying magnifications. 



This method appears to be the most scientific that has yet 

 been suggested for estimating exposure ; but here, again, the 

 same objection applies that it is left to the discretion of the 

 worker to determine what allowance shall be made for colour 

 or density differences in his object. With the same illuminant, 

 however, and with one that can always be worked at the same 

 intensity, it would appear to be of considerable service. 

 It is based on the assumption that critical illumination is 

 being used, and is not applicable if this condition is departed 

 from. 



If any determination of the actinic value of the projected 

 image is to be made, then the most practical method is to 

 employ either a Wynne or Wat kins exposure -meter. This is 

 the method employed in ordinary pictorial photography, and 

 under proper conditions may be used with considerable accuracy 

 even in photo-micrography. It depends on observing the 

 actual effect of the light on paper coated with a sensitive 

 emulsion. The paper is allowed to darken until its tint 

 exactly matches that of a tinted standard supplied with the 

 appliance. For photo-micrography the type that is usually 

 made for estimating the exposure when doing ordinary 

 photographic enlarging, is the one to employ. Using this in 

 conjunction with a source of light of moderate or high power ^ 

 such as lime-light or the electric arc the actinic value of the 

 light may be determined with accuracy. In this case also some 

 trial exposures would need to be made, and from such trial 

 exposures a table may be drawn up for future reference. The 



