Page:Principles of Microscope.djvu/33

 Rh We may consider the types of pictures above enumerated from these different points of view.

The respective advantages and disadvantages of the different types of pictures from the point of view of the disclosure of elements which subtend a comparatively small angle at the retina will come clearly before the eye on turning to Plate I and setting up the book at progressively increasing distances from the eye. It will be seen that resolution will be lost in Figs. a and b, where the elements are delineated by outlines, sooner than in Figs. c, d and e, where the elements are represented respectively, by patches of light and shade, by patches of colour, and by patches of colour enclosed in outlines.

Consideration will bring out the reason of these differences.

In outline pictures, such as are furnished by Figs. a and b, the elements remain visible only so long as the diameter of the delineating outline subtends the necessary angle upon the retina.

In pure colour pictures, such as are shown in Fig. d, the elements remain visible as long as their total diameter subtends that necessary angle.

In the case of outline and colour pictures and in the case of pictures in relief, resolution will be lost somewhat sooner than in the pure colour picture by reason of the fact that the area of colour, or, as the case may be, of illuminated or shaded surface, is in each case less than the total area of the object.

What holds true in the case of these figures holds true also generally. It holds true in the case of the bull's eye of a distant rifle butt. Here an advantage in visibility is secured by the substitution of a solid bull's eye, which subtends a comparatively large angle upon the retina for a bull's eye of similar dimensions defined only by an outline. It holds true also in connexion with the microscopic work where objects such as bacteria, which subtend even under high magnification only a small angle at the eye, are to be brought clearly into view.

Subject to the reservation which is made below with respect to the combined outline and colour picture, all the types of picture