24 INTRODUCTION. 



between the most diverging rays issuing from the axial point 

 of an object that may enter and take part in the formation 

 of an image. By axial point is meant a point situated in the 

 extended optical axis of the microscope. 



The optical axis of the microscope is a line drawn from the 

 eye through the middle of the tube to the centre of the 

 objective. 



Relation of Size and Working Distance of the Lens. The 

 larger the lens and the less its working distance, the greater 

 the angle of aperture. For dry objectives the greater the 

 angular aperture, the better the definition of the objective. 



Numerical aperture is the capacity of an optical instrument 

 for receiving rays from the object and transmitting them to 

 the image, and the numerical aperture of a microscopic objec- 

 tive is, therefore, determined by the ratio between its focal 

 length and the diameter of the emergent pencil at the point 

 of its emergence that is, the utilized diameter of a single- 

 lens objective or of the back lens of a compound objective. 

 It is the ratio of the diameter of the .emergent pencil to the 

 focal length of the lens, or, in other words, it is the index of 

 refraction of the medium in front of the objective multiplied 

 by the sine of half the aperture. 



The Ocular Lens or Eye-piece. 



The eye-piece, or ocular, is the lens, or combination of 

 lenses, placed in the tube at the point of observation. It 

 acts as a simple microscope and serves to magnify the 

 image of the object. The ocular consists of two lenses, 

 one -situated nearer the eye, known as the eye-lens, and 

 the other known as the field-lens. The ocular is said to 

 be positive when the image is formed beyond it ; and nega- 

 tive when it is formed within it, between the field-lens and 

 eye-lens. In the positive ocular the two lenses act together 

 as a simple microscope and magnify the image. In the 

 negative ocular the field-lens acts with the objective in 

 making clearer the image, and with the eye-lens in help- 

 ing to correct some of the aberrations. The eye-lens also 

 magnifies the image. 



