144 



SMITH, ON MICROSCOPIC ILLUMINATION. 



easily and firmly on the end of the object-glass, their length 

 being adapted to the focus and form of the latter. When 

 brought down upon the slide under examination, they slip 

 back readily to allow of adjustment, and completely exclude 

 light from the upper surface of the object. 



Light-modifier. — Some apparatus attached to the micro- 

 scope is required for the purpose of diffusing and purifying 

 light. It should admit of easy change from one kind of 

 modification to another during the examination of an object, 

 and without having to withdraw the eyes. The following 

 contrivance suggested itself to me, and answers the purpose 

 extremely well. Cut a sector of a circle of convenient size 

 out of a piece of sheet brass, and make three holes, centred on 



the circumference of a circle concentric with the first, a short 

 distance apart, each hole equal to the largest aperture of the 

 diaphragm of the microscope. Fit a short slit tube at the 

 angular point, at right angles to the plate, and having its 

 central axis passing through the centre of the larger circles 

 first mentioned. The tube should fit closely on the round 

 stem of the body-support beneath the stage and above the 

 mirror. Be careful to take the radius of that circle which 

 passes through the centres of the three holes, so that -when 

 the plate is moved from right to left, or vice versa, each hole 

 shall in turn coincide, with the large aperture of the dia- 

 phragm. Solder three rings exactly round the three holes, 

 a little larger than they, to form a ledge for the reception of 

 the glass circles next to be described. Let in and secure, 

 with gold-size or other cement, three circles of plane glass ; 

 one white ground, for diff"using ordinary daylight ; a second 

 neutral tint ground, for diffusing lamp-light or strong sun- 



