256 SUPPLEMENT TO THE 



doublet, or visual pencil in an engiscope, to the centre of 

 the image of the perforation in it. An engiscope, treated 

 with this system of illumination, shews, in the margin of 

 its visual pencil, a round image of the diaphragm, but, in 

 plano-convex, single, or compound magnifiers, with their 

 plane sides towards the object, this image assumes a semi- 

 lunar figure. The intensity of the illumination is regu- 

 lated by the distance of the image of the perforation, or 

 of the focus of the illuminating lens, from the plane of 

 the object ; and if the light of a southern window, on a 

 bright day, is employed, the instrument will be found to 

 perform best, when it is made equal to about half or three- 

 fourths of an inch ; the intercepted light in this method is 

 divergent. The modification of Dr. W.'s illuminator, 

 contrived by me, which has a real diaphragm, instead of 

 the image of one, has an advantage over the original con- 

 struction, in being more portable, and in adapting itself 

 easily to any construction. 



13. There are certain classes of objects which can 

 never be seen completely well with artificial light of 

 any kind, unless the instrument be perfect, such are 

 animalcules and aquatic insects, the hairs of a variety 

 of animals, mosses, the tissue of confervse, and an in- 

 finite number of common objects. When it is neces- 

 sary to exhibit them by candlelight, they are best seen 

 with the flame very close to them, as already described ; 

 or if the construction of the instrument admits not of this 

 arrangement, then, by rendering the illuminating rays 

 parallel, by a bull's-eye lens, and afterwards reflecting 

 them by a plane mirror, and using diaphragms under the 

 stage, any condensation, by destroying the parallelism of 

 the rays subverts the effect of the bull's-eye lens, and will 



