244 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
the same direction (direct) or in the opposite (inverse). Accordingly, 
if the observer move along the length of the pencil, so long as he is in 
front of, or beyond the extreme points of crossing of the rays, the path 
of the light in the whole extent of the displacement will be either direct 
or inverse. On the other hand, throughout the extent of the zone 
limited by these extreme points, there will be simultaneously direct and 
inverse paths for a certain number of positions of the small hole during 
a transversal displacement. 
The study of chromatic aberration was made in the same manner, 
the bright regions presenting the colour of the rays isolated by the 
small hole when white light was used. 
The longitudinal aberration proving very troublesome to measure, 
owing to its enormous extent, the author confines himself to the 
transverse. 
The object was a luminous slit, traced with a sharp razor in a silver 
layer deposited on a glass plate, and had a width of from 0 • 005 mm. 
to 0*0025 mm. The slit was covered by a cover-glass. The small 
hole, having a diameter of 0 * 8 mm., was displaced perpendicularly to 
the slit, and the displacement was read on a scale giving 1/10 mm. 
The Microscope was adjusted so that the image was clearly defined in 
the plane of the small hole, whose distance from the slit, placed on the 
stage, was always the same, 0 * 20 mm. 
The image was generally decomposed into two parts, one with rays 
coming from the marginal zone of the objective presenting an aberration 
in a determinate direction, the other, corresponding to the central zone, 
presenting an aberration in the opposite direction. In subtracting from 
the total displacement (between the limits of appearance and extinction 
of the light) the displacement due to the central zone, the same result 
was always obtained as in taking account of the dimensions of the slit 
and hole measured directly. 
The objectives of the best makers of France and Germany were 
studied, and gave measurable spherical aberration varying from tenths 
of a millimetre to several millimetres. In measuring the spherical 
aberration, a coloured glass was placed over the small hole; for the 
chromatic aberration white light was used, and its value was judged by 
the intensity of the variations of the tints. Nearly all the objectives had 
sensible chromatic aberration, but many were found in which it was 
scarcely perceptible, and in some, constructed simply of flint and crown 
glass, it was quite inappreciable. 
The author concludes, therefore, that the problem of achromatism 
may be considered as solved, but that that of aplanatism is far from 
being so. For the improvement, therefore, of objectives, the correction 
of the spherical aberration must Ido chiefly kept in view. 
(6) Miscellaneous. 
Dr. Hudson’s Presidential Address.— The ‘Times’ of the 18th 
February contained the following leading article on this Address. 
“ An address such as the President, Dr. C. T. Hudson, delivered 
before the Royal Microscopical Society at its Annual Meeting last 
week is as surprising as it is delightful. All science has a tendency to 
grow more and more technical and elaborate. So complicated become 
