MICROSCOPIC OBJECTS. 35 



scope, the fluid has a tendency to flow towards the pupil, and injure 

 the distinctness of vision. 



4. If the microscopic object is longitudinal, like a fine hair, or 

 consists of longitudinal stripes, the direction of the lines or stripes 

 should be towards the observer's body, in order that their form may 

 be least injured by the descent of the lubricating fluid over the 

 cornea. 



5. The field of view should be contracted, so as to exclude every 

 part of the object, excepting that which is under his immediate ex- 

 amination. 



6. The light which is employed for the purpose of illuminating the 

 objects, should have as small a diameter as possible. In the day time 

 it should be a single hole in the window-shutter of a darkened room, 

 and at night it should be an aperture placed before an argand 

 lamp. 



7. In all cases, and particularly when very high powers are 

 requisite, the natural diameter of the light employed should be 

 diminished, and its intensity increased by optical contrivances. 



23. As the whole subject of the illumination of objects is of the 

 most important character, I shall make no apology for quoting the 

 remarks of an able microscopist on the subject.* u Much of the 

 beauty," he remarks, " of the objects seen depends upon the manage- 

 ment of the light that is thrown upon or behind them, which can only 

 be fully mastered by practice. It may be remarked, however, as a 

 general rule, that in viewing those which are transparent, the plane mir- 

 ror is most suited for bright daylight, the concave for that of candle or 

 lamp-light, which should have the bull's-eye lens, when that is used, so 

 close to it, that the rays may fall nearly parallel on the mirror ; if the 

 bull's-eye lens be not used, the illuminating body should not be more 

 than three inches from the object, the details of which are usually best 

 shown when the rays from the mirror fall upon it before crossing ; and 

 the centre should, especially by lamp-light, be in the axis of the body 

 of the microscope. For obscure oltjects, seen by transmitted light, 

 and for outline, a full central illumination is commonly best ; but for 

 seeing delicate lines, like those on the scales of insects, it should be 

 * Mr. J. Smith, ' Microscopic J ournal/ vol. i. p. 48. 



