200 THE MICROSCOPIST. 



nating apparatus should be perfect, than that the magnifying 

 one should be so ; and the essential part of his method consists 

 in this : That the rays which form the illuminating image or 

 disk shall have their foci exactly on the part of the microscopic 

 object to be observed, so that the illuminating rays may radiate 

 as it were from the object, as if it were luminous. Now this 

 can only be well attained by illuminating with a single lens, 

 or a system of lenses, without spherical or chromatic aberra- 

 tion, whose focal length, either real or equivalent, is less than 

 the focal length of the object-glass of the microscope. The 

 smaller the focal length of the illuminating lens, or system of 

 lenses, the more completely do we secure the condition, that 

 the illuminating rays shall not come to a focus, either before 

 they reach the object, or after they have passed it. 



MODE or OBTAINING THE WHEEL ANIMALCULE (Vortir 

 cella fotatorla). '" Early in the spring I fill a three-gallon jug 

 with pure rain-water (not butt-water, because it contains the 

 larvae of the great tribe). This quantity more than suffices to 

 fill a half-pint mug, and to keep it at the same level during the 

 season. I then tie up a small portion of hay, about the size 

 of the smallest joint of the little finger, trimming it so that it 

 may not occupy too much room in the mug, and place it in the 

 water ; or about the same quantity of green sage leaves, also 

 tied and trimmed. About every ten days I remove the decay- 

 ed portion with a piece of wire, and substitute a fresh supply. 

 A much greater number of animalcules are raised by the sage 

 leaves ; but I have sometimes been obliged to discontinue the 

 use of it, on account of its producing mouldiness. I take them 

 out with an ear-picker, scraping up the sides of the mug near 

 the surface (including the dirt which adheres to them by the 

 tail), or under the hay or sage/' J. Ford. 



SUBSTITUTE FOR THE CONCAVE SPECULUM. Mr. G-. Jack- 

 son employs a plano-convex lens of about two inches in diame- 



