738 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
by a suitable glass. Tbe aperture is said to be 1*30. The objective 
is to be used with compensation eye-pieces, and for the tube-length of 
160 mm. should give magnifications respectively of 600, 900, 1200, 1800, 
2700 with the eye-pieces 4, 6, 8, 12 and 18. 
As the result of his examination of the system, the author determines 
the focal length as 1 • 8 mm., and the numerical aperture measured with 
the apertometer as 1 • 32. 
When used as eye-piece with the front lens turned towards the eye, 
the system shows no large spots and not too many small ones. 
The objective stood the Abbe test very well. With central illumi- 
nation, when an immersion condenser of 1 * 40 N. A., and the compensation 
eye-piece 6 were used, the edges of the slit of the test-plate in the central 
parts of the field of view were very sharp and perfectly free from colour- 
fringes and mistiness, while on the periphery small colour-fringes were 
visible, and, quite on the edge, a slight mistiness. With the slightest 
changes in the adjustment the colour-fringes of the secondary spectrum 
make their appearance. The image is sharp and free from colour up to 
the extreme edge of the field. With the compensation eye-piece 12, the 
edge of the eye-piece diaphragm has a light orange-yellow fringe. 
With oblique illumination including rays inclined 60° to the axis, 
the colour-fringes of the secondary spectrum are very distinct, but no 
mistiness is observed. By blocking out the most oblique rays and 
diminishing the effective aperture to about 1 * 20, the colour-fringes are 
reduced to a minimum, and the two edges of the slit appear very sharply 
defined up to the extreme edge of the field. A test of the objective 
by means of a scale of Pleurosigma and also of tubercle-bacilli gave very 
good results. 
The resolving power corresponds perfectly to the aperture 1 * 32. 
With central illumination the cross-striations of Surirella gemma are 
very distinct. With intense lamp-light and oblique illumination the 
objective showed the cross-striation of Amjpliijgleura jpellucida. With 
oblique monochromatic illumination by means of sunlight and a 
CS 2 -prism these striations appear in the limit between the green and 
blue (A = 0 • 48 /a) of the spectrum ; in blue light (A = 0 • 45 /a) they are 
very distinct, and in the blue-violet (A = O' 42 /a) indications of the 
pearl-structure are visible. In order to test the optic homogeneity and 
elasticity relations of the material of the lenses, the behaviour of the 
objective in polarized light was examined. The lenses were found to 
be perfectly homogeneous and isotropic, for no signs of double-refraction 
were noted either with parallel or with convergent polarized light. 
(3) Illuminating and other Apparatus. 
Clay Wick for Microscope Lamps.* — The indestructible clay wicks 
give 25 per cent, more light than cotton wick. They are made in any 
desired shape or size and have been used for high-power work in some 
laboratories for months. They require no trimming or attention and the 
wicks do not clog. These wicks are made by arranging vegetable fibres 
in the unbaked clay, so that when fired a series of longitudinal pores are 
left through which the oil is raised by capillary attraction. Owing to 
* Amor. Mon. Micr. Joum., xv. (1894) pp. 30-1. 
