592 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
allows focussing if the short-focus “ planar ” or Dallmeyer lens be 
used. 
Among other details are some special directions for the microphoto- 
graphy of culture tubes. 
The formula F = ^ ^ ^ is found to be sufficiently accurate for 
practical work (F = focal length of lens required, M = magnification, 
and L = camera-length). ' Thus, if it be required to produce a magni- 
fication of four diameters with a camera - length of 10 in., then 
F = — jp - = 2 in. In the same way, if any two of the three quan- 
tities F, L, M be known the third can be calculated. 
(2 and 3) Medium and High-power Work. — In these sections the 
various difficulties to be encountered are fully described. The author’s 
favourite Microscope is the Zeiss model I A, although apparently it 
is so badly balanced that it has to be clamped to the table to prevent it 
tumbling over. 
(5) Microscopical Optics and Manipulation. 
Microscopic Images and Vision.* — Mr. Lewis Wright, from the 
point of view of the microscopist, continues the investigation of this 
subject on the lines commenced by Lord Rayleigh and Dr. Stoney.f 
His object is to show that the “ spectrum ” (i.e. the ordinary Abbe) 
theory is only true in a conditional and limited sense, while its accept- 
ance in a universal sense is a present cause of positive mischief in 
microscopy. He begins by discussing some fundamental physical 
objections to Dr. Stoney’s seven propositions, which propositions may 
be summarised thus : — (A) “ All light emitted by an object may be 
resolved into undulations consisting of uniform plane waves.” (B) “We 
may conceive these reversed in direction (since any dynamical system 
may be reversed) ; and when they thus arrive back at the position 
occupied by the original object, they will then produce an image the 
most perfect that the light emitted is caj>able of producing.” Hence in 
general, “plane waves converging inwards” are capable of producing the 
most perfect attainable image producible from the rays grasped by the 
objective. Mr. Wright objects that uniform plane weaves are not in 
trustworthy microscopy the actual dynamical system, and therefore 
cannot, as he afterwards shows they do not, produce the supposed most 
perfect attainable image by reversal. More specifically, it seems evident 
that we are debarred from considering the light from a microscopic object 
as consisting of uniform plane waves, except on the condition of plane-wave 
illumination of the object. (Here, indeed, we have the secret of Abbe’s 
consistent enforcement of illumination by a small luminous cone or 
pencil, which gives approximately such illumination.) To get the 
normal wave-surface approximately plane, the beams of rays must be 
got approximately parallel, which is attained in physical optics by 
removing the source of light, itself relatively small, to a considerable 
distance from the grating or other apparatus. Now, in the case of a 
* Phil. Mag , June 1898, pp. 480-503 (1 fig.). 
f Cf. this Journal, 1896, p. G81, and 1897, p. 71. 
