174 THE MICKOSCOPE. 



For those who desire to obtain a very oblique illumi- 

 nating pencil, with objectives of lower powers, bui^. large 

 angular aperture, and with an inexpensive arrangement, 

 the Hemispherical Condenser (Kettledrum) of the Eev. 

 J. B. Eeade will be found serviceable. 1 This gentleman 

 now, however, uses a double hemispherical condenser for 

 the examination of the most difficult tests ; and the great 

 obliquity of the illuminating pencils gives every facility 

 for dealing with the close fine lines of the Amician test 

 and the Macrum. Diaphragms of tinfoil are placed 

 between the two hemispheres. By looking down the 

 body of the microscope when the eye-piece is removed, 

 and examining the dimensions of the little discs of light, 

 it is seen at a glance whether one or other of the aper- 

 tures require to be more or less deeply cut. In the one 

 case, the little lappet of tinfoil can be so doubled as to 

 shorten the aperture, and, in the other, it may be cut 

 deeper and thrown further back. To obtain these very 

 slight but not unimportant variations, and in a moment, 

 is an advantage which observers will readily recognise. 

 Mr. Eeade also uses more durable diaphragms of thin 

 brass, and, by -a new arrangement, the precise amount of 

 light is obtained at once. A fixed diaphragm, with aper- 

 tures at right angles to each other, and cut nearly to the 

 centre, is placed on the lower hemisphere ; and another 

 diaphragm, of which the outline is a right angle upon a 

 semicircle, is placed in a slit of the tube between the 

 condensers, so that the vertex of the right angle divides 

 the space between the long V apertures of the lower 

 diaphragm. When pushed home, it nearly shuts up these 

 apertures of the fixed diaphragm ; but by gently drawing 

 it out, and moving it a little sideways, if necessary, we 

 can obtain, with the utmost nicety, just that length of 

 either aperture which the test-lines under examination 

 require. 



The very perfection of oblique illumination for resolving 

 the markings on test-objects, is the double prism made 

 use of by Mr. J. Newton Tomkins. The method of employ- 

 ing the two prisms we give in our friend's words : 



(1) Rev. J. B. Reade, F.R.S. on a New Hemispherical Condenser. Trans. 

 Micros. Soc. 1861, p. 59, and vol. vii. page 3, 1867. 



