172 Mr Lister on the improvement of the 



they will be found to be much enlarged by diminishing the 

 aperture of the object-glass. 



They are enlarged also, without contracting the aperture of 

 the glass, by increasing the intensity of the illumination, where- 

 as by darkening the object beyond a certain point they may 

 be rendered ill defined, and be at length dissolved. 



These peculiarities are most observable on some opaque ob- 

 jects, (the reflection from a very small microscopic globule of 

 quicksilver offers perhaps the best example,) but the same ef- 

 fects are produced on the light received from transparent ones ; 

 and the consequent blunting and mingling together of their 

 minute details when the object-glass admits but a small pen- 

 cil of light, gives rise to various fallacious appearances. One 

 of the most remarkable is the spottiness which some surfaces 

 assume, not unfrequently so much resembling small globules 

 as to have been mistaken for them ; an optical illusion having 

 thus been the basis of some ingenious speculations on organic 

 matter. 



Such appearances have little place with the finer achroma- 

 tics, the large angle of whose pencil and its accurate correc- 

 tion enable us to magnify their image greatly, still discovering 

 something new in our objects, before we are checked by the 

 circles of diffusion of the effective rays ; but these at last, whe- 

 ther proceeding from the causes mentioned or from others to 

 be hereafter noticed, form in every microscope the boundary 

 to defining power, except where faulty materials or workman- 

 ship give it an earlier limit. 



It is the marginal rays which contribute especially to ren- 

 der visible close and delicate lines, such as those on the scales 

 of lepidopterous insects, and some of the most difficult of these 

 are even best seen when the central light is intercepted. — (See 

 Note A.) 



A glass that is far from correct in its figure will sometimes 

 show lines of this description sharply, while the outline of the 

 scale is indistinct, and the contrary ; but one in which both 

 aberrations are destroyed should give the outline and the lines 

 distinct together. Even some good glasses, however, have a 

 defect, against which we should be on our guard ; that in ccr- 



