The Illumination of Objects. 49 



a transparent description, and viewed by transmitted light, can 

 only differ from each, other by showing colour, by transmitting 

 light in different proportions, by refracting light with different 

 degrees of force, or by polarizing it, and thus rendering it in- 

 capable of transmission through certain bodies in all directions. 

 We shall omit the question of polarization, and notice the other 

 actions mentioned. Where difference of colour exists, it is ver}? - 

 important in enabling us to distinguish one part of a structure 

 from another. Coloured objects should be examined by white 

 light, and care should be taken not to let it be so strong as to 

 obliterate delicate tints. Varying degrees of transmissive 

 power j and varying degrees of refractive power, operate within 

 certain limits in much the same way. The most transparent 

 portions of an object let most light through, and the least 

 refractive bend the least light out of its straight course to our 

 eyes. Thus the chief effect of contrast between more and less 

 transparent parts, and more or less refractive parts of any 

 object will be analogous — namely, that of stopping a greater or 

 less transmission of light to the eye. Refracting bodies exert 

 a power of dispersing or separating white light, which consists 

 of rays of all sorts of colours, into distinct colours, according 

 to their several refrangibilities. When this power is strongly 

 exerted, very positive colours, as in the prismatic spectrum, 

 appear ; when feebly exerted, a slight difference of tint is 

 noticeable, and that is all. 



Suppose we have to examine a beautiful diatom, such as 

 Arachnoi discus, or a live transparent object, such as a rotifer, 

 our object is to illuminate the one or the other, so as to enable 

 nice gradations in refracting or absorbing power to be seen. 

 The first thing to be done is to send the light in a good direc- 

 tion, and the second to regulate its quantity, and vary the 

 quantity within certain limits either way. If a lamp is placed 

 as far off an ordinary bulFs-eye condenser as its focal length, 

 it will transmit parallel rays — that is to say, its power of bend- 

 ing rays will just compensate or bring straight the divergence 

 of the rays from all parts of the lamp that will fall upon its 

 surface. Such parallel rays may be thrown upon a flat mirror, 

 and sent up straight through our object. We have then illu- 

 mination in its simplest form. We must apportion the quan- 

 tity of light to the delicacy of the object ; and if this is done, 

 we shall see that some parts allow more than others to reach 

 our eye. Careful experiments should be made in varying the 

 quantity of light transmitted without changing its direction. 

 The easiest mode of doing this is by means of an elegant piece 

 of apparatus lately introduced by Mr. Collins ; we mean his 

 " Graduating Diaphragm." An instrument of this kind 

 appears to have been made long ago by Dollond ; but Mr. 



VOL. VIII. — NO. I. E 



