ILLUMINATION BY REFLECTION 



329 



the prisms, open the slit somewhat, and use the apparatus as a 

 simple ocular. If one has to deal with a small object which does 

 not entirely fill the slit, but allows rays of light to come in past 

 it and disturb the spectrum, he should turn the comparison prism 

 so as to shut up some of the slit, without, however, letting in the 

 light upon it, and then bring the object up near to it, and from the 

 other side push up the shortening apparatus as close as is necessary. 

 On the other hand, should the object consist of a number of single 

 minute grains, which would cause to be drawn across the spectrum, 

 in the direction of its length, perpendicular to the Fraunhofer lines' 

 a like number of dark lines, 

 one must adjust the micro- 

 scope so that the object will 

 be a little out of focus, some- > 

 what above or below the true' 

 focus. In this way we shall 

 get a uniform spectrum. The 

 spectrum can also be improved 

 in some other cases by like- 

 wise throwing the object 

 somewhat out of focus. 



Illumination by Reflec- 

 tion. Objects of almost every 

 description \vill require at 

 times to be examined and 

 studied by what is called re- 

 flected light ; the light in 

 this case is thrown down upon 

 the object by various devices, 

 and is reflected upwards 

 through the objective. This 

 has been called ' opaque illu- 

 mination,' which, however, 

 is not a comprehensive, nor 

 even an accurate designation. 

 Only a small proportion of 

 the objects examined in this 

 way are opaque ; the same 

 diatom, for example, may 

 often with advantage be ex- 

 amined with transmitted 

 light, being transparent, and again by means of an illumination 

 thrown upon, and reflected up from, its surface ; also a condenser 

 with a central stop, when used for a dark ground, shows objects by 

 reflected light, but it is manifestly not ; opaque illumination.' The 

 designation of this method of illumination is consequently more 

 accommodating than accurate. 



There are two very simple means of obtaining this superficial 

 illumination when low powers are employed. The first is the 

 ' bull's-eye ' (which is nowhere in this work called a ' condenser ; ' 

 this would, as it often has done, lead to confusion ; it is enough to 



FIG. 277. The English form of bull's-eye. 



