1 8 PHOTO-MICROGH APH Y 



When it is used in the reversed manner, it certainly has given very good results, 

 as shown in Fig. 5, Plate I. ; still, however, if the magnification required was over 

 ii to 2 diameters, it was only with difficulty we found ourselves able to reach the 

 milled head to focus with. 



Messrs. Dallmeyer, however, have constructed a small rectilinear of about if-mch 

 focus, which with magnifications over to 2 diameters has produced most admirable 

 results, and were it only constructed to work equally well in red, green and violet 

 rays, we should have no possible fault to find with it, as it is very inexpensive. 

 Examples of its performance are shown in Figs, i and 2 and 7, Plate I. But the 

 finest type of lens with which we are acquainted has been recently introduced by the 

 firm of Zeiss, and called by them the " Planar." It has been constructed from com- 

 putations by Dr. Rudolph for the especial purpose of photo-micrography, and it works 

 equally well in all colours. See Figs. 3 and 4, Plate I. 



There remains yet one more point concerning the selection of lenses for certam 

 sized objects, which relates to what is called their " Covering power," about which at 

 present nothing has been said. How it applies to the subject under consideration is 

 not altogether easy to point out, and cannot be found described as such in any text- 

 book with which we are familiar. But as it is of such practical importance to the 

 photographer it may be well to try and explain the matter. 



By the "size of plate covered" is meant that if a lens is used in the ordinary 

 manner, say upon distant objects (to get parallel rays), the diameter of the plate 

 covered at the focus on the ground glass is of definite dimensions. That is to say, all 

 objects outside this limit are ill-defined, and the light falls off more and more until the 

 illumination becomes almost nil To see how this applies to our subject we must 

 treat the matter by regarding the rays in the opposite direction, i.e., the area of 

 defined illumination of a given object which the lens can cast on the plate is limited 

 to the size of the covering power already described. Say with the 50 mm. lens. 

 Its area of defined illumination is limited to about i j by ij, hence it cannot be 

 used to magnify objects larger than by li, for the image on the ground glass will 

 fall off so that the edges are imperfect ! If therefore the object he larger than aoouc 

 I inch by I inch, there is no alternative but to use another lens of greater covering 



power. ^ 



This is exactly what happened in photographing the Frontispiece. The Cardinal 

 Beetle was just beyond the limit of area for the 50 mm. Zeiss Planar, hence we had to 

 employ a Boss-Zeiss Planar, of different focus (83 mm.) and larger covering power 

 (kindly placed at our disposal by the firm) to take the photograph. But as the Ant- 



