36 VIEWING AND ILLUMINATING 



made to fall obliquely, and in a direction at right angles by the lines to 

 be viewed. 



" The diaphragm is often of great use in modifying the light and 

 stopping such rays as would confuse the image ; but many cases occur 

 when the effects desired are best produced by admitting the whole 

 from the mirror. 



*' The most pleasing light for objects in general is, that reflected 

 from a white cloud on a sunny day ; but an argand lamp or wax candle, 

 with the bull's eye lens, is the best substitute." 



24. TRANSPARENT AND OPAQUE OBJECTS. It is generally known 

 that objects for the microscope are sometimes prepared as transparent, 

 at others as opaque objects. In the former instance the light reflected 

 from the mirror is made to traverse, or rather to pass directly through 

 the object ; it is, in fact, viewed by transmitted light. In the latter, 

 the light is thrown directly on the object. This may be accomplished 

 in one or other of two ways. By one method a lens fixed to the stage 

 is made to collect the rays of light, and thus to direct them on to the 

 object ; in the other the light is thrown up from the mirror on to 

 the metallic specula attached to the object-glasses, (called from their 

 inventor Lieberkhuns,) from which is reflected into the object. A large 

 proportion of opaque objects are seen perfectly well (especially by day- 

 light) with the side illumination; and for showing irregularities of sur- 

 face, this lateral light is sometimes the best ; but the more vertical 

 illumination of the Lieberkhun is usually preferable, the light thrown 

 up to it from the mirror below being, with good management, suscep- 

 tible of much command and variety. 



25. ON VIEWING OBJECTS BY POLARIZED LIGHT. The addition 

 of a polarizing apparatus to a microscope is an excellent accompaniment 

 to the instrument, whether we consider its peculiar properties, or the 

 brilliant colours it gives to all bodies affected by it. Mr. Pritchard 

 contrived a very simple apparatus for this purpose ; it may readily be 

 attached to or removed from the microscope, without either disturbing 

 it or the object under consideration. It consists of two small tubes 

 containing single-image calcareous* prisms, or plate of tourmaline, and 

 it is to be used in the same manner as plain diaphragms. 



