276 PEACTICAL PHOTO-MICKOGKAPHY 



required would be nearly 2 mm. In practice it is found 

 advisable to shift the object by the required amount rather 

 than to shift the objective. The degree of necessary separation 

 may be determined by ruling two lines on the focussing-screen 

 at a suitable distance apart, and then moving the object 

 from side to side so that the centre of the field of view coincides 

 with the lines that are ruled on the screen. The exposures are 

 made in succession as in taking ordinary photographs. The 

 objectives most suitable are those of the Zeiss planar type, 

 as their field is flat and their aperture is such that the maximum 

 penetration is attained. 



The majority of suitable objects are best illuminated by 

 reflected, rather than by transmitted, light. Any source of 

 light may be used, and exposure will vary within exactly the 

 same limits as in ordinary photo-micrographic work. The 

 illuminating beam may conveniently be a convergent one. 

 There are no unusual practical difficulties to be encountered in 

 this class of work. It is essential to see that in exposing, the 

 two pictures receive exactly the same effective exposure ; and 

 that in development the same treatment is given exactly to 

 each. If the two pictures are on the same plate the latter 

 requirement would be complied with in the ordinary course. 

 Where succeeding photographs are taken, it is perhaps best 

 not to employ a high-power source of light, like the electric arc, 

 as there is often considerable variation in the constitution of the 

 carbons, and the actinic power is therefore not always exactly 

 the same for a given period. It is also more difficult with 

 high-power illuminants to give precisely the same exposure, as a 

 very slight variation would cause an appreciable difference in the 

 two negatives. Practically, therefore, it is better to use a source 

 of light such as the Nernst lamp, the mercury- vapour lamp, or 

 acetylene, which is constant, and which entails an exposure of 

 moderate length. 



The Production of Lantern-slides in Colour. The pro- 

 duction of lantern-slides in colour has now become such a 

 recognised method, and is, under certain conditions, of such 

 utility, that a short description of the principal available 

 methods should be given. It is, however, only a few objects to 

 which the process is applicable or becomes necessary. With the 

 majority of stained objects, although it increases their appear- 



