334 SCIENTIFIC APPARATUS. 



ments which are not absolutely necessary than to have any badly 

 carried out. A roughly-constructed yet complicated instrument 

 is not fitted for practical use. As an almost universal rule, cheap 

 instruments are better to be simple. Assuming that the work is 

 carried out in a satisfactory manner, it does appear desirable that 

 the tube of the microscope should not only be easily moved up 

 and down over a considerable space with power of accurate move- 

 ment for fine adjustment with high powers, but that there should 

 at all events also be the means of using the instrument either 

 vertically, horizontally, or inclined at any convenient angle. A 

 vertical position is not only inconvenient, but the very worst, 

 for correct definition. 



Very much the same general principles will apply in the case 

 of the stage. It is very convenient to be able to move the 

 objects up and down by rack or screw work, and to be able to 

 3 otate the stage whilst they always remain in the centre of the 

 field ; but if the movements are not well carried out in practice, it 

 is better to have an immovable stage, and to trust to the hand 

 alone in adjusting the object. Very much, however, depends on 

 the nature of the work for which the microscope is designed. 



* 



ILLUMINATORS. 



The proper illumination of objects is of the very highest 

 importance, and that suitable for one class may be altogether 

 unfit for another. Though there may not be any exact line of 

 division between them, yet it is convenient to divide the methods 

 of illumination into three principal divisions ; viz., that by 

 reflected, that by transmitted, and that by refracted light. 



For opaque objects, reflected light must be used, and even 

 when an object is sufficiently transparent to admit of the use of 

 transmitted light, some characters are better seen by light reflected 

 from the surface. Except for very low powers, this surface illumi- 

 nation must be made sufficiently bright by means either of so- 



