248 Transactions of the Royal Microscopical Society. 



by similar treatment ; many of them also give the black boundary 

 ring, and the white disk above the plane of that ring. 



If we remember how exceedingly colourless is a film of black ink 

 the 50,000th of an inch thick (placed between two glasses), we can 

 at once understand that a minute spherule of organic matter may 

 naturally be also colourless, and be capable of forming a spurious 

 disk : in no case have I ever been successful in forming a solar disk 

 less than the 60,000 th of an inch in diameter under the microscope ; 

 although theoretically it ought to have been optically a miniature 

 of a millionth of an inch. 



Bright Hghts wll always form large disks of minute refracting 

 spherules, and this is often a source of much obscurity. 



In viewing the intercostal spaces of Podura, which are generally 

 hlanTc in most microscopes, or at best in the largest specimens cross- 

 barred, I have found a test of correction of the greatest value to 

 render these spaces brighter and brighter by the screw-coUar and 

 searcher, when at last I have been rewarded with the development 

 of a rich beaded structure in these blank intercostal spaces. But 

 in all cases the so-called spines are found in a focal plane between 

 the upper and lower set, Mr. R. Beck's " out of focus and out of 

 adjustment " ribs engraved on his plate being a real approximation 

 to true structure. The black shadow illuminator first constructed 

 by me in 1864, finely develops the spherical shadow-phenomena.* 



The instrument consists of a strong brass slide for the sub- 

 stage, mounted very firmly with an axis carrying an object-glass. 

 A Ross' 1^-inch with a shield is generally preferred. The angle of 

 inchnation employed is generally about 15"' or 20°. 



The wide-angled achromatic condensers are so egregiously full 

 of spherical aberration as to be incapable without stops (and even 

 with them) of producing pure shadows jetty black so necessary for 

 deUcate investigations. 



When a pinhole stop is placed above these wide-angled con- 

 densers, the cap comes so close to the slide as to endanger the 

 glasses of high powers. The angular aperture is then reduced to 

 about 10^. Any object-glass such as an inch stopped off behind or 

 before produces precisely the same effect with the double advantage 

 of superior aplanatism and a safe convenient working distance. 



* The highest degree of sliadow is produced by stopping off exactly one-half 

 of the front lens of the illuminator, viz. that half which is nearest to the axis of 

 the microscope, and in some cases a U-shaped aperture greatly heightens the 

 effect, the top of the U being turned away from the axis. 



