442 E, M. NELSON, NOTES ON EXHIBITS. 



First you will notice that it is in black-dot focus, the silex 

 being white. The black dot is the correct focus, for the white 

 dot is a diffraction ghost, a function of the unutilised annulus of 

 an object-glass. As this unutilised annulus is diminished by the 

 enlargement of the axial cone the white dot vanishes away. In 

 brief, white dots and narrow cones are synonymous terms. 



Narrow-cone photomicrographs, showing white dots, are v^ery 

 easy to take (not more difficult than ordinary landscapes, for 

 photomicrographic difficulties only begin with the large cone, and 

 are then difficulties of a very serious nature), and when done are 

 no test of the objective with which they were taken ; this, there- 

 fore, has been photographed with a wide cone, and the magnifica- 

 tion is X 4,000 diams. A Watson achromatic oil-immersion 

 condenser was used immersed with all the photographs. Kindly 

 note the fracture at the side of the cap ; there are minute 

 spikes which have been broken, four long and two short, while 

 in the bridge piece the fractured spikes are of different lengths, 

 and on the other side there are no long spikes at all. This very 

 delicate structure may not be visible on the sheet, though on the 

 lantern slide it is quite plain. 



It will be in the remembrance of some present that in 1886, 

 when Abbe introduced the apochromatic lens, he stated that in 

 the high powers there were some outstanding errors which could 

 not be removed in the objective itself, and that it was necessary 

 to use an over-corrected (compensating) eye-piece to correct those 

 errors. Further, in order that all the series of apochromatic 

 objectives should be used with the same eye-pieces, intentional 

 errors were introduced in the lower powers. Therefore it was 

 the microscope as a whole that was apochromatic, and not the 

 objective itself. 



Now all this has been altered by Messrs. Leitz, who have pro- 

 duced a really apocliromatic ^V, the performance of which is 

 injured by the use of an Abbe over-corrected eye-piece. 



If you examine the new compensating eye-pieces of Messrs. 

 Leitz you will find the accustomed orange fringe at the edge of 

 the diaphragm is no longer there. The microscopical world is all 

 the richer for the introduction of these beautiful eye-pieces of 

 Messrs. Leitz, as well as of the fine complanats of Messrs. Winkel. 



Journ. QueLett Microscopical Club, Ser. 2, Vol. XL, No. 70, April 1912. 



