54 THEMICROSCOPIST. 



which they are mounted. If the object is small, and so mount- 

 ed as not to intercept much of the light from the mirror, the 

 mode illustrated by Fig. 16 is the best ; in other cases, that 

 shown in Fig. 15 is preferable. 



The transmission of light obliquely, as near as possible at 

 right angles to the axis of vision, which is recommended in the 

 foregoing extract, for viewing delicate lines, has a very fine 

 effect, the field of view being dark, while the objects are bril- 

 liantly illuminated. The lines on the most difficult test objects, 

 as some species of Naviculae, &c., are to be seen in this manner 

 of illumination, even when otherwise invisible. For producing 

 this oblique illumination in the best manner, several ingenious 

 pieces of mechanism have been invented ; see page 42. 



Next to the proper illumination of the object, the adjustment 

 of the focus is the most important thing to be attended to. 

 With a low power, the coarse adjustment is usually sufficient 

 if the workmanship be good ; but with a high power it becomes 

 necessary to resort to the more delicate arrangement of the fine 

 adjustment. Great care must be taken, however, lest the glass 

 on which the object is mounted be broken, or the object-glass 

 injured, by too sudden or too close a contact. 



