Prof. H. W. Dove on a New Photometer. 17 



after the analysing Nieol has been placed in the aperture of the 

 objective tube. 



To diminish the aperture, a metallic lineal can be used in 

 which circular apertures of gradually diminishing size are ar- 

 ranged in a straight line, of which the largest has the same 

 diameter as the aperture. I shall call this lineal the slider. 

 In the lineal which I used there were fourteen such apertures. 

 Their diameter can be determined by the micrometric arrange- 

 ment of the microscope. Instead of the slider an eccentric 

 disc can be used with diminishing apertures, as is often found on 

 the older microscopes; yet a rectilinear slider is preferable, 

 because the excentric disc, if it is to contain many apertures, 

 becomes of an inconvenient size. The distance of the source of 

 light is determined on a scale. The zero-point of the scale is, in 

 a horizontal position, the photographic picture, which is fastened 

 on the objective stand by the ordinary objective holder. 



In order to determine the size of the acting surface, any 

 angle-measurers may be used. If the body to be tested is a 

 plane surface diffusing light, it can be placed in the centre of a 

 horizontal circle by whose limb it can be moved; or a plane 

 mirror is placed on the surface, in which a distant scale is read 

 off by means of a telescope. If the intensity of the light inci- 

 dent on a plane mirror under different angles is to be determined, 

 the same method can be used to determine its angle. If, on 

 the contrary, the reflecting plane is the free surface of a liquid, 

 the incidence is obtained by inclining the axis of the microscope, 

 which is measured by means of a mirror fixed to it. In a similar 

 manner the rotation of the analysing Nicol is obtained by means 

 of an affixed mirror. 



Feeble sources of light can be intensified, if in a vertical posi- 

 tion, by changing the plane illuminating-mirror for a concave 

 mirror, and in a horizontal position by an illuminating lens 

 which is so fixed that the concentration on the photographic 

 image takes place in the same manner as previously by the con- 

 cave mirror. Where parallelism of the incident rays is required, 

 the source of light is in the focus of the lens. 



For different sources of light the process is different. I shall 

 discuss it singly for the different kinds. 



Dioptric Colours of Absorption and diffused Light of Transparent 



Bodies. 



Coloured Glasses. — When the microscope is in a vertical posi- 

 tion, the object is illuminated from below by the mirror directed 

 towards a portion of the heavens, and from above by the ordinary 

 light of day. The thickness of the glasses which close the aper- 

 ture of the objective is now changed until compensation is 



Phil. Mag. S. 4. Vol. 25. No. 165. Jan. 1863. C 



