242 SUMMARY OF CURRENT RESEARCHES RELATING TO 



work, escapement and compensation balance. The size of the mirror is 

 2\ by 4 in. Fig. 30 is an instrument with Heele's modifications. The 

 size of the mirror is 3 by 5 in. Fig. 31 is of simpler form. The clock- 

 work is contained in a mahogany case. The instrument is fitted with 

 spirit-level and levelling screws. Fig. 32 is of still simpler construction. 



D owdt, S. E. — Microscope condenser fitting. 



[Describes how an effective condenser can be cheaply improvised.] 



English Mechanic, Ixxix. (1904) p. 50. 



(4) Photomicrography. 



Photographing Microscopic Crystals.* — W. Bagshaw shows that 

 a combination of transmitted and reflected light is necessary to throw 

 objects like microscopic crystals into relief and impart a pleasing and 

 faithful representation. The transmitted light, subdued so as not to 

 dominate the reflected light, ensures the outlines in their finest rami- 

 fications, whilst the reflected light casts the shadows. 



The How and Why of the Lippmann Colour Process.! — T. A. 

 O'Donohoe reminds his readers that the Lippmann film is usually a 

 very thin transparent film of gelatin containing a very small proportion 

 of perfectly emulsified silver bromide. The glass support must be 

 between the film and the lens, and the film must be backed by mercury 

 to form the reflecting surface. Suppose fig. 33 to represent a section 

 of the film ; A B the glass surface in • contact with the film, and C D 

 the mercury, also in contact with the film. Let R be a ray of mono- 

 chromatic light passing through the film in a sinuous unbroken line, 

 and impinging at right angles on the surface of the mercury. At the 

 moment of reflection it loses half a wave-length, and according to 

 Young its phase is reversed, so that it returns in the form of the dotted 

 sinuous line, interfering more or less in its course with the entering 

 wave. The two systems of waves are now, as it were, locked np in the 

 film, and are called " stationary waves," because they have lost their 

 forward motion and can only move up and down within the film. 

 They rise and fall with incredible rapidity and act chemically, all the 

 time producing the greatest effect where their motion is greatest, and 

 the least or no effect in the nodal planes where the two waves intersect. 

 In the figure the planes of highest chemical activity are represented by 

 lines max, and from these to the shorter lines min, where there is no 

 chemical effect, there is a gradual waning of actinic power. There are 

 thus alternate planes parallel to the mercury, showing the maxima and 

 minima of chemical action, and should the theory be correct, a trans- 

 verse section of such a film should, after development and fixation, show 

 these maxima and minima by alternate bands of black, where the deposit 

 of silver bromide is greatest, and of white bands, where the deposit 

 of silver bromide is little or none. Other colours of the spectrum will, 



* Amateur Photographer, xxxix. (1904) p. 69 (4 figs.). 

 t Photogram, x. (Sept. 1903; pp. 271-4 (6 figs.). 



