THE MICROSCOPE. 19 



form at B B a real image of a dimension capable of being 

 seen through the upper lens, termed the eye-glass, E E. 

 A diaphragm or stop B B, consisting of a blackened metal 

 disc with a hole in the centre, cuts off the extraneous rays, 

 and thus gives sharpness to the image. 



29. The Faults of a Lens. There are two faults in 

 every lens which must be overcome before a satisfactory 

 image of an object can be produced. These faults are of 

 trivial importance with very low magnifying powers, but in 

 the case of high powers they become so serious, if not 

 corrected, that the microscope is useless. The faults are : 

 i. Spherical, 2. Chromatic aberration. 



30. Spherical aberration is due to the manner in which 

 rays are refracted by a spherical surface. The axial ray A 

 (Fig. 22), striking the lens at a right angle, is not refracted. 

 The rays C C around the axis are bent to a slight extent 



FIG. 22. Spherical aberration. 



and brought to a focus at c ; the lateral rays R R, striking 

 the lens much more obliquely, are focalised at r ; conse- 

 quently the image of an object is rendered indistinct, its 

 margins r r are misty and ill-defined, owing to the lateral 

 rays having already passed their focal point. 

 _ 31. Spherical aberration is to a large extent remedied 

 by intercepting the lateral rays with a diaphragm. This 

 method is often had recourse to in simple microscopes, 

 but it has the disadvantage of diminishing the aperture 

 of the lens, and thereby causing a loss of light. In the 

 eyepiece of the compound microscope, however, where 

 the lenses are large, a diaphragm is used (B B, Fig. 2 1). 



