MICROSCOPICAL IMAGERY. 15 



effects as imaged by the diatomic bead. Here are a few examples. 

 Fig. I, central discs; Fig. 2, ex-centric; Fig. 3, others taken on 

 the opposite side. All these images were found to be above the 

 diatomic bead, proving it to be convex. 



A more recherche experiment is to exhibit the image of stops in 

 either P. and L. condenser or Gillett's. These stops are sup- 

 ported by short arms, and vary in sizes, and produce intensely 

 jet-black images amid a blaze of reflected light. Sometimes, a 

 plane mirror only is used without a condensing lens, the movable 

 stops rotating i7i situ, P. and L. having three only : Nos. 1,2, 

 and 3. The stops are ^-inch, ^-inch, and ^-inch in diameter; 

 the }^ stop, as miniatured by the lens of a beetle's eye, appears 

 about I — 5,000th of an inch in diameter. 



But less difficult of manipulation is the mullion of a window 

 12 inches broad and about 130 inches distance. Each facet of 

 the lens shows a vertical bar, whose thickness in imagery is easily 

 measured by a good micrometer.* This mullion appeared 78,000 

 times smaller, and optically calculated gave a focal length of 

 I — 600th of an inch. The facet lens is i — i, 000th of an inch 

 across, and the refractive index 3/2, so that after all the animal 

 possesses glass eyes ! (The formulae are too intricate for insertion.) 



The 12-inch mullion t image, 120 inches distant, in a 

 spherule of that splendid diatom, Creswellii siiperba, measured 

 i-6o,oooth of an inch. Consequently^ the miniature is 1-7 20,000th 

 smaller, and yet the mullion was distinctly visible, whilst the 

 distance of the image above the convex surface was just 

 I — 6,000th of an inch. 



{To be contiiiued.) 



EXPLANATION OF PLATE 11. 



Figs. 1, 2, and 3.— Imagery of a minute illuminated hole below the 

 condenser, shown with direct ; and right and left oblique 

 lights. 



,, 4 and 5.— Images of a P. and L. stop of condenser, \ inch in 



diameter, as seen in large diatomic beads. 

 ,, 6. — Beetle's eye, showing stop images in each facet. 



* Mine, with 500, reads to two niillionths. 

 t A vertical sash-bar, carrying the balance weights and cords. 



