and on certain Appliances for Illumination. 113 



out, my having to remove the condenser, except the doublet-lens, 

 which I have made to take out, without unscrewing, through the 

 large aperture of the revolving stage itself. This simple arrange- 

 ment enahles me to pass from one kind of illumination to another, 

 or from the upper to the lower stage, with the greatest facility. It 

 is well known that the condensing lens, which is a help with a §-inch 

 or -^-inch objective, especially if used binocularly, may be worse than 

 useless for lower powers, say a 2-inch, except when required for dark- 

 ground illumination ; and ought therefore to be capable of easy 

 removal, independently of the perforated diaphragms, which are 

 always wanted ready in their place. 



Secondly, my purpose in adding a sub-stage was to gain racking 

 space for the use of a 4-inch or 5-inch objective, and at the same time 

 to provide for the quick resort to the polarizing apparatus whenever 

 it might seem desirable, or to the light-modifier, which is such a 

 comfort when a lamp is employed. The sub-stage slides by a short 

 tube upon the bottom tube of the main stem of the stand, and has a 

 deep notch which firmly nips the square part of the stem and keeps 

 everything duly centred. The brass plate, on which objects are 

 supported by a sliding fork and bar, is circular, of the same dia- 

 meter as the revolving stage above, and concentric therewith, having 

 itself a central aperture 1-^ inch across. Three diaphragms turn one 

 under another upon a fixed pivot below the plate, each diaphragm 

 having two apertures rather more than | inch in diameter. These 

 apertures are furnished with shallow revolving cells, and one of 

 them carries the polarizer. The shape of the diaphragms is some- 

 thing hke the sector of a circle, and is such as to avoid needless 

 weight, while allowing the whole set to be turned aside, so as to 

 leave the large opening of the sub-stage quite clear. The upper 

 edge of each diaphragm is the segment of a circle, and shdes 

 smoothly past a catch-spring which serves for a chck. The dia- 

 phragms, each having two apertures, are fitted as follows: — the 

 one nearest the mirror with a Nicol prism as polarizer, and a light- 

 modifier of pure blue glass ; the second with a plano-convex lens 

 and a neutral selenite ; and the third with two selenites respectively 

 blue and yellow, and red and green. All revolve by means of milled 

 collars. They can be combined in various ways with ease, as each 

 selenite may be used alone, or the neutrals may be combined with 

 either of the other two, or all may be turned aside, and the polarizer 

 alone left, or hght-modifier, as may be required. The plano-convex 

 lens appears to improve the performance of the polarizer with 

 low powers ; and it suggests other uses which I have not yet 

 worked out. 



Thirdly, it still remained to provide for the analyzer. A short 

 Nicol prism seemed on the whole the best, if only it could be 



