44 MICROSCOPIC ILLUSTRATIONS. 



with combinations of short foci than with shallow lenses ; 

 so much so, indeed, that 60 or 70 degrees with a focal 

 length of l-10th of an inch is gained with as great facility 

 as 20 degrees with an inch focus. Thus, in proportion 

 to the minuteness and delicacy of an object, the focus 

 must be shortened, if we would have a suitable penetra- 

 tion ; otherwise, the labour and time expended upon cor- 

 recting the pencils of light, more especially the oblique 

 ones, will be immense. The best proportions of angular 

 aperture to focal length are treated of in another part of 

 this work. 



About the time of Dr. Goring's applying achromatics 

 to the microscope, M. Sellique, in France, invented and 

 executed a new description of object-glass, part of the 

 arrangement of which, on account of the many advantages 

 it possesses, is, and will probably continue to be, adhered 

 to by opticians. Its novelty and chief merit were, that it 

 consisted of several pairs of corrected lenses ; each pair 

 having a plano-concave of flint glass, and a double convex 

 of plate glass, cemented with mastic varnish. The latter 

 of these lenses was turned towards the object ; by reason 

 of which, as well as from the want of angular aperture, 

 the combination was not so effective as it was afterwards 

 rendered. The genius of M. Chevalier, of Paris, soon 

 led him to perceive some of these defects. He saw at 

 once that the plan of combining thin pairs of lenses 

 greatly diminished the chromatic dispersion, and he saw 

 also that the placing the convex lens next the object must 

 necessarily increase the aberration from sphericity. He 



