EXPLANATION OF PLATES 



FKtfNTISPJECE 



Fig. 1. x 6 diameters. Horizontal and transverse section of an orbitolite. 



Fig. 2. An imperfect or uncritical image of the minute hairs on the lining 

 membrane of the extremity of the proboscis of the blow-fly x 510 diams., taken 

 with a Zeiss apochromatic ^-inch objective of '95 N.A. x 3 projection eye-piece ; 

 but it was illuminated by a cone of small angle, viz. of 0-1 N.A., and illustrates 

 the unadvisability of small cones for illumination. 



The first obvious feature in the picture is the doubling of the hairs which 

 are out of focus ; but the important difference lies in the bright line with a 

 dark edge round the hairs which are precisely in focus. This is a diffraction 

 effect which is always present round the outlines of every object illuminated 

 by a cone of insufficient angle. Experiment shows that this diffraction line 

 always ceases to be visible when the aperture of the illuminating cone is equal 

 to about two-thirds the aperture of the objective used : but it will become 

 again distinctly apparent when the aperture of the cone is reduced less than 

 half that of the objective. 



Fig. 3. x 510 diams. A correct or critical image of the minute hairs on the 

 lining membrane of the extremity of the blow-fly's proboscis. In this picture 

 the focus has been adjusted for the long central hair. It will be observed that 

 this hair is very fine and spinous ; it has not the ring socket which is common 

 to many hairs on insects, but grows from a very delicate membrane, which in 

 the balsam mount is transparent. This photograph was taken with a Zeiss 

 apochromatic of -95 N.A. x 3 projection eye-piece. The illumination was 

 that of a large solid axial cone of '65 N.A. from an achromatic condenser, the 

 source of light being focussed on the object. 



Fig. 4. Section of cerebellum of a lamb, x 77 diams., by apochromatic 1-inch 

 3 N.A. This preparation was courteously supplied to the present Editor by Dr. 

 Hill, whose imbedding and staining processes for these tissues it beautifully 

 illustrates. 



Fig. 5. Amphipleura pellucida x 1860 diams., by apochromatic ^ 1'4 N.A. 

 illuminated by a very oblique pencil in one azimuth along the valve. 



Fig. 6. A hair of Polyxenus lagurus, a well-known and excellent test object 

 for medium powers x 490 diams. by apochromatic % '95 N.A. 



Fig. 7. A small vessel in the bladder of a frog, prepared with nitrate of 

 silver stain, showing endothelium cells, x 40 diams., by Zeiss A. -2 N.A. This 

 object has been photographed for the purpose of exposing the fallacy which 

 underlies the generally accepted statement that ' low-angled ' glasses are the 

 most suitable for histological purposes. The supposition that it is so has 

 been founded on the fact that the penetration of a lens varies inversely as its 

 aperture ; therefore, it is said, a ' low-angled ' glass is to be preferred to a 

 wide-angled one, because ' depth of focus,' which is supposed to enable one to 

 see into tissues, is the end in view. 



On carefully examining this figure it will be noticed that it is almost 

 impossible to trace the outline of any particular endothelium-cell because its 

 image is confused with that of the lower side of the pipe. In a monocular 

 microscopical image a perspective view does not exist ; it is better, therefore, to 

 use a wide-angled lens, and so obtain a clear view of a thin plane at one time, 

 and educate the mind to appreciate solidity by means of focal adjustment. It 

 will be admitted that unless one approaches fig. 7 with a preconceived idea of 



