Page:Principles of Microscope.djvu/31

CHAPTER I.

GENERAL CONSIDERATIONS WITH REGARD TO THE OBJECT PICTURE.

Definition of the object picture—Classification of pictures as outline pictures, colour pictures, outline and colour pictures, and pictures in relief respectively—Preliminary consideration of the respective advantages and disadvantages of the different varieties of pictures enumerated above—Advantages and disadvantages of the different varieties of pictures from the point of view of the disclosure of objects which (by reason of their minuteness or distance) subtend only a small angle at the retina—Advantages and disadvantages of the different types of pictures from the point of view of the disclosure of the configuration of the object in plan and relief—Advantages and disadvantages of the different types of pictures from the point of view of the disclosure of the internal structure of the object, Advantages and disadvantages of the different types of pictures from the point of view of their greater or less liability to be vitiated by fallacy.

1. Definition of the object picture.

We may designate by the term object picture that system of radiant points which forms the original of the image which is produced on the retina. Where an object is viewed by the microscope, the term stage picture may conveniently replace the more generic term object picture.

2. Classification of pictures as outline pictures, colour pictures, outline and colour pictures, and pictures in relief respectively.

Object pictures may be classified in conformity with the following considerations :—

(a) Objects may be defined by outlines alone. They may, according to circumstances, be delineated by black outlines on a bright field, or by bright outlines on a dark field. The corre- sponding pictures may be spoken of as "outline pictures."^[1]

(b) Objects may be defined against a background by a difference of colour. The corresponding pictures may be spoken of as "colour pictures."

1. This is the type of picture which Koch has, not altogether felicitously, designated the Strukturbild—structure picture.