or THK ancRoscorE. 153 



The rays must, of course, be parallel. If either divergent or 

 convergent, some of the rays will be oblique. Purely axial or 

 central illumination can be obtained only by passing the light 

 through a very small hole placed some distance below the stage. 



Oblique Light. Many objects fail to show their peculi- 

 arities when illuminated by parallel rays of light passing 

 through them in the direction of the optic axis of the micro- 

 scope, but are seen very clearly when the light is sent through 

 them obliquely. To secure illumination by oblique light re- 

 flected from the mirror, the latter must be so suspended that it 

 can be turned to one side, and thus send a beam of light through 

 the object at an acute angle. Where direct light is employed, 

 the necessary degree of obliquity may be obtained by adjusting 

 the position of the lamp a device to which we have resorted 

 when compelled to use a stand in which the mirror did not 

 swing to one side. In this way, also, oblique light may be em- 

 ployed to illuminate objects viewed through a pocket lens, and 

 very interesting effects obtained. For the resolution of fine 

 markings upon diatoms, etc. , oblique illumination is a neces- 

 sity. When the angular aperture of the objective is low, and 

 the light is very oblique, the objects appear light on a dark 

 ground in fact a sort of dark ground illumination is obtained. 



The Achromatic Condenser. The earlier forms of 

 the achromatic condenser consisted simply of an achromatic 

 lens, similar to an object-glass, so arranged that by means of it 

 the light from the mirror could be brought to a focus on the 

 object. With some objects, even this simple contrivance gave 

 very fine results. It was soon found, however, that great ad- 

 vantage was derived from cutting off portions of the pencil of 

 rays transmitted by the condenser, and by means of the proper 

 diaphragms, central, peripheral and one-sided or oblique illum- 

 ination was obtained. First-class achromatic condensers be- 

 came, therefore, quite complicated and expensive. Several 

 cheaper but very efficient forms are now made by opticians, a 

 favorite being the Webster condenser, shown in Fig. 45. 



Of this accessory Carpenter gives the following very practical 

 description: "In its present form the arrrangernent of the 



