MICROSCOPES. 203 



naturally dark field, show the structure of the object "by light 

 thrown on them from below, bent out of its normal course by 

 more or less oblique reflection and refraction. This may be 

 accomplished in various ways. The light may be thrown on 

 the object by the mirror from one side in such a manner that 

 if no object were on the stage it would not pass at all up the 

 body of the microscope. But if the object placed on the 

 stage is granular, or has little facets and other irregularities 

 of structure, the light would be so bent as to pass up to the 

 eye. This may be accomplished also by means of two or 

 three different kinds of illuminators, such as Amici's prism, 

 Mr. Wenham's reflex illuminator, and his parabolic reflector, 

 or by a large lens of short focal length, with a central black 

 spot to stop out direct light. All these different kinds of 

 illumination depend upon the light being thrown so obliquely 

 from the under side that it will not enter the object-glass 

 unless it be turned out of its course by the reflection or the 

 refraction of the various parts of the object. One great 

 advantage is that the field of the microscope is dark, and the 

 eye is not in any way distressed by the light which comes in 

 at the sides. Certain kinds of structure are also seen to very 

 great advantage, especially with binocular instruments. I am 

 inclined to believe that the further development of this kind 

 of illumination would yield better results than can be obtained 

 by the ordinary method of illumination by transmitted 

 light. 



Another kind of illumination which is on the whole more 

 common, is whers the light is reflected directly up the tube 

 of the microscope, so that if no object be placed on the 

 instrument the field of the microscope is filled with light, 

 and if any object be there it is seen by variation in the 

 intensity or colour of the light. I wish to draw your 

 attention to one or two points connected with this kind of 

 illumination, because, in my opinion, its further development 

 with certain modifications which I believe have not been 

 carried out, will perhaps enable us to overcome certain 

 difficulties which at present stand in our way. Much may 

 be learned by the study of mineral structures, since in the 

 case of crystals and of solid portions of glass and other 

 analogous objects, we know what their character is, whereas 

 in the case of minute organic structures we have rather to 

 infer what is their structure from what we see. Therefore in 



