48 Aids to Microscopic Inquiry. 



AIDS TO MICKOSCOPIC INQUIRY. -No. VI. 



The Illumination op Objects. 



The right method of illuminating microscopic objects is a most 

 important subject of inquiry. The beginner finds one of his 

 chief difficulties in the arrangement of illuminating apparatus, 

 and the most practised and scientific manipulators are con- 

 tinually occupied with questions only differing in degree from 

 those which puzzle the student at the commencement of his 

 career. 



The object of illumination is of course to show all that the 

 glasses employed can render visible, and to do so in such a 

 manner as to make microscopic vision as easy and as little 

 fatiguing as the ordinary exercise of the unaided eye. The 

 beginner usually finds his arrangements in excess on one side 

 or the other. Either his object is too much in the dark to be 

 distinctly visible, or his field is flooded with so much light as to 

 distress the eye, and render it impossible to discover delicate 

 structure. The more experienced microscopist avoids these 

 errors, but is apt to indulge in pet methods of illumination, and 

 to assume, from appearances presented under peculiar circum- 

 stances, that he knows all the microscope can tell him concern- 

 ing the objects he surveys. 



It is well at the outset of microscopic experiments to inquire 

 what effects illumination can produce. It is obvious that we 

 only see objects by means of their action upon light, and we 

 can learn whether they reflect it or transmit it, allow it to pass 

 straight through them, or bend it out of its course. If we 

 illuminate them with white light, and see them coloured, it is 

 plain that they have prevented certain rays from reaching our 

 eyes either by interception or absorption, or by sending them 

 in another direction. 



From this statement of the results that become visible when 

 objects are under illumination, we may proceed to the conside- 

 ration that if an object is of a composite structure, we may treat 

 it so as to show the kind and degree of action which its several 

 parts can exert upon light under different circumstances. 

 When the microscope is employed to investigate any structure, 

 we must arrange the illumination so as to test it in various 

 ways. Each mode of illumination may teach us something not 

 visible under another method, and by putting the results of 

 various methods together, we arrive at conclusions concerning 

 structure that either could not be reached at all, or not safely 

 reached from any single method of optical investigation. 



Apart from their form, the various parts of any small object of 





