DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 139 



trate it more fully by means of the accompanying" en- 

 gravings, so that any one may make trial of it, and 

 thence form a correct judgment of its merits. I should 

 mention, that the effects produced are not so striking in 

 some objects as in others, and therefore that it is worth 

 while to make trial of several kinds. 



Fig. 29 represents one of my instruments : its body, 

 eye-piece, and object-glass, n,i,e; the spring double- 

 stage, /; the mirror, o, which is turned aside ; the candle- 

 holder, k, I; and jointed condenser, m. In this arrange- 

 ment it is proposed to illuminate the object by an arti- 

 ficial condensed light. Let the object-slider be placed 

 between the stage and the circular plate; adjust to the 

 focus, and place the candle, /, and condenser, m, in the 

 positions shewn in the drawing, so that the pencil of rays, 

 r, r, may form the required angle with the axis of the 

 instrument, i, i. This angle will be dependent upon the 

 angle of aperture of the object-glass; but in all cases it 

 must exceed half that angle, in order to produce the 

 desired effect, otherwise the field of view will be illumi- 

 nated. But if a small transparent object, such as the 

 pollen of a flower, the scale of an insect, &c. be placed 

 within the field of view, the rays impinging upon it will 

 be refracted, so as to enter the microscope, and they 

 will form an illuminated image of it upon the dark 

 ground. 



When this experiment is conducted by day-light, the 

 instrument should be placed in the position shewn by 



