OBSERVING WITH AND MANAGING ENGISCOPE. 199 



to steam the lenses. The arm, w, is made to turn round 

 in any position most convenient to the observer. The 

 adapter (fig. 28), which is to be used between the bars of 

 the slider-holder, to carry a small aquatic live-box, or the 

 like, affords another species of convenience of a similar 

 nature, and serves to elevate the face of the observer 

 above the stage. 



Opaque Objects. Opaque objects seldom require such 

 high powers, or such large apertures, as transparent ones. 

 Their colours are best brought out by daylight, natural 

 or condensed ; but artificial light, perhaps, shews them 

 best in all other respects. 



They should always be viewed upon a black ground of 

 some description. The more sombre, and dingy, and 

 faint, their tints are, the greater need there is of a black 

 ground to stifle all heterogeneous light. 



As they only reflect or radiate light, a much stronger 

 illumination is requisite for them than for diaphanous 

 bodies, which almost always transmit a great deal more 

 light than opaque ones are capable of reflecting. More- 

 over, they seldom present flat surfaces, as most pellucid 

 bodies can be made to do. This, when the power is con- 

 siderable, causes one part of the object to be indistinct, 

 while another is seen accurately : this must always be 

 allowed for. A very perfect instrument can only have 

 one point in focus at once, and, consequently, can only 

 shew one point distinctly at once, all the rest not on the 

 plane of the focus being confused. 



