514 SUMMARY OF CURRENT RESEARCHES RELATING TO 



ing power ; tte latter was also increased by interposing a blue-green 

 glass, but diminished by the use of red and orange-yellow glasses. 



Dr. Flesch concludes, as the result of his experiments, that the 

 incandescent light excels every other artificial light for clearness and 

 brightness of field and for steadiness. He is opposed to any plan of 

 fixing the lamp to the stand of the instrument, better results being 

 obtained when the lamp is placed immediately below the condenser 

 than when the light is reflected by a mirror. 



Mayer's Black-ground Illuminator.* — This is a simple form of 

 black- ground illuminator, devised by Prof. A. M. Mayer, for the study 

 of aquatic life with low-powers of aperture up to 60°, showing aquatic 

 organisms as brilliant objects on a black ground, so that they are 

 instantly detected among the more opaque particles of ooze. The 

 interior structure of rhizopods, infusoria, rotifers, worms, &c., is also 

 brought out in a manner which is said to be very striking. With dark- 

 ground illuminators which give large angles to the emergent pencils, 

 the interior structure of translucent bodies is not so well seen. 



The optical combination consists of three plano-convex lenses in 

 contact with one another, which the author denotes as A, B, and C, in 

 their order from below upward. A is a plano-convex lens with its 

 plane side facing the mirror ; the radius of its curvature being 

 21 in. and its thickness 0'175 in. B and C are plano-convex lenses 

 with their convex sides down ; radius 1 in. and thickness 0*4 in. 

 On B is cemented a stop, formed of a piece of paper blackened with 

 lamp-black in shellac. The diameter of the central stop is 0-71 in., 

 and the width of the annular opening round the stop • 1 in. 



Each of the lenses in the experimental form of the illuminator 

 exhibited had a diameter of 1^ in. It is evident that this diameter 

 may be lessened in the lenses B and C, so that the combination when 

 mounted will have the form of the frustum of a cone. With this form, 

 the combination could enter the aperture of the majority of stages, 

 and its upper lens be brought even in contact with the under side 

 of the slide. 



The mean angle of the emergent rays at the upper lens C is 

 69^°. The mean diameter of the annular opening of the stop is 

 calculated in reference to the curvatures of the lenses, so that the 

 central rays issuing from this stop fall normally on the convex surface 

 of the lens C, and thus traverse it without refraction. This also tends 

 to correct the chromatic dispersion of the pencil of rays emerging 

 from B, whose boundaries of red and blue fall in directions inclined 

 towards the normal of the lens C, on opposite sides of this normal. 



The plane mirrors, as generally made, of nearly all Microscopes, 

 except those of the large models, are too small in the front and rear 

 diameter to illuminate the lower lens of dark-ground illuminators ; 

 and the author obviates this defect by cutting an ellipse out of a 

 piece of plane mirror, and attaching this to the frame of the ordinary 

 mirror. The ellipse has a mirror axis a little larger than the diameter 

 of the lower lens of the illuminator, and the major axis is so long that 



* Jouin. New York Micr. Soc, ii. (1886) pp. 28-30. 



