MICROSCOPES. 333 



advantages are secured, but the power of converting the instru- 

 ment at once into a direct simple microscope is lost. The great 

 merits of binocular instruments are that the eyes are less fatigued, 

 and the difference in the level of different parts of an object are 

 far more easily appreciated than with monocular. Solid objects 

 illuminated by surface light are seen in well-marked relief, am 1 

 even in the case of thin flat sections of transparent substances 

 the general structure is much more readily understood than when 

 only one eye is used. It is also far less trying to the eyes to use 

 both at a time. When only one is employed it suffers from over- 

 work, and the other from underwork, and there is too great a 

 contrast between the use of the eyes when looking through the 

 microscope and when looking at other objects in the ordinary 

 manner. With high powers these advantages are, however, often 

 more than counteracted by loss of light and by imperfect defini- 

 tion over a large part of the field. It is therefore very desirable 

 to be able at pleasure to remove or insert the prism, and employ 

 the instrument monocularly or binocularly, according as circum- 

 stances make one or other form most suitable. For certain kinds 

 of work it may be better to forego this power of binocular 

 vision in order to secure other advantages, when the total cost is 

 of importance. 



STAND AND STAGE. 



A most important requisite is to have a microscope so con- 

 structed that movements affecting it may equally influence both the 

 object-glass and the object ; since if either could move in the 

 smallest degree independent of the other, slight and unavoidable 

 tremors would interfere in a most objectionable manner with the 

 use of high magnifying powers. Certain movements are essential 

 or very desirable, but all should be of such a character and so 

 thoroughly well carried out in practice as to withstand a consider- 

 able amount of wear and tear without the separate parts becoming 

 loose and giving rise to tremor. It is far better to have no move- 



