226 SUMMARY OF CURRENT RESEARCHES RELATING TO 



normal spectrum, which is again focused by means of an achromatic 

 condensing lens F upon either the vertical illuminating apparatus of the 

 Rosenhain Microscope, upon the surface of an object, or upon the mirror 

 of an ordinary Microscope. It is generally advisable to interpose a 

 screen with a broad slit in the focus of this lens if but one coloured 

 light is required, especially if the bench is being used to illuminate a 

 substage condenser for transmitted light. Such a screen is most con- 

 veniently carried on the Microscope itself, but in the case of the vertical 

 illuminator of the Rosenhain Microscope it is made to slide into the 

 fittings of the body. 



The above describes the bench as used for monochromatic illumina- 

 tion, but for ordinary illumination the diffraction grating and prism E, 

 with the condenser F and the slit C, are removed. An iris diaphragm 

 and a water cooling chamber G may be used, and if desired a 4-in. 

 lantern condenser may be used instead of the condenser B. A mirror 

 on carrier H may be supplied to fit the bench. A lantern carrier and 

 projection lens convert the whole apparatus into a most convenient and 

 serviceable optical lantern. 



An incandescent gas lamp may be supplied in place of the Nernsb 

 lamp. 



(4) Photomicrography. 



Method for Determining the Exact Colour for Light Filters. — 

 E. Moffat communicates the following easy method for determining the 

 exact colour of screen required to photograph successfully a faintly 

 stained slide of Bacteria, etc. Place some crystals of chlorate of potash or 

 salicin (the crystals of which give a good range of colour) under a 

 Microscope, and examine by polarised light. Revolve one of the prisms 

 till any one of the crystals matches exactly the stained preparation : now 

 turn the prism round 90°, when the. complementary colour will appear. 

 This will show the exact tint required for the screen, and will produce 

 the greatest amount of " darkness " or contrast, e.g., bacilli stained 

 faintly blue will require a very dark orange screen, or if stained faintly 

 red, a dark green will be required ; the lighter the stain, the darker 

 must be the screen, and vice versa, in order to produce a sharp image on 

 the photographic plate. Hence the saving of time in using the polari- 

 scope to determine the required depth of tint. Isochromatic plates 

 must be used in conjunction with these screens. To prepare the screens 

 fix unexposed dry plates in the dark room and stain with any suitable dye. 



(5) Microscopical ( Optics and Manipulation. 



Artificial Double Refraction, due to inotropic Distribution, with 

 Application to Colloidal Solutions and Magnetic Fields.* — T. H. 

 Havelock, after reviewing the methods of artificially obtaining double 

 refraction, and after investigating the theory of their formation, sum- 

 marises the sections of his paper thus : — 



1. The formal investigation of artificial double refraction in colloidal 

 solutions as due to a deformation of the medium, consisting of a change 

 in the packing of the colloidal particles. 



* Proc. Roy. Soo., Series A, lxxvii. No. A 515 (Feb. 1906) pp. 170-82. 



