Prof. H. W. Dove on a New Photometer. 19 



able glass plates, is also effected when the instrument is in a 

 horizontal position. The microscope is employed in the same 

 position when the liquid is one which transmits very little light, 

 as, for example, indigo solution. For this purpose I have used 

 the capillary apparatus, in which the liquid ascends between two 

 slightly inclined glass plates. The distance from the edge gives 

 the increasing thickness of the layer of liquid for a given distance 

 of the plates. In the case of coloured gases which are enclosed 

 in vessels, the instrument also stands horizontal. The turbidity 

 by smoke may be investigated in the open air. 



With cloths, paper, &c, the increasing thickness is obtained 

 by folding them together. The quantity of light transmitted 

 in this case is not that directly observed, but partly that trans- 

 mitted by the interstitial spaces. The same is the case with 

 thin coatings of gold or silver. 



Measurement of the Light diffused by Opake Bodies. 



If, in ordinary daylight, a sheet of white paper is held hori- 

 zontally under the objective tube of a microscope placed horizon- 

 tally, the illumination from above can be so regulated that the 

 dark inscription is seen upon a white ground. By increasing 

 the inclination, the white ground appears continually brighter 

 in reference to the dark inscription. If the white sheet is 

 exchanged for a dull black one, or for a uniformly blackened 

 surface, under all inclinations the white inscription appears on a 

 dark ground. With a coloured surface it is different. If in 

 this case the bright inscription appears upon a dark ground, at 

 a certain inclination it disappears, and, on increasing the incli- 

 nation, changes into a dark inscription upon a bright ground. 

 This gives a very simple means of judging which of two adjacent 

 colours is the brightest. The surface is inclined until the 

 inscription disappears, and the other colour is then brought 

 into the same position. It is brighter or darker according as 

 the inscription appears black or white. In an accurate deter- 

 mination of the angle at which the transition takes place, all 

 lateral light must be excluded. To effect this, I have placed the 

 horizontally arranged microscope in such a manner that a tube 

 a foot long, blackened on the inside (part of a gun-barrel), was 

 in the continuation of the optical axis of the microscope, by 

 placing this tube directly against the inferior aperture of the 

 objective tube. The coloured discs, placed in a vertical position 

 and working on a vertical axis, were placed at the other end of 

 the tube. 



The moment at which a colourless transparent body, such as 

 water, becomes opake in consequence of total reflexion, i3 

 distinctly seen by looking obliquely into a glass at the under 



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