428 Address by the President. [Sess. 



rays will be about three inches from the centre of the mirror. 

 Divergent rays, such as the rays from a lamp, increase this 

 distance. The rule for the calculation of the distance is : 

 Multiply together the radius of the mirror and the focal 

 distance of the radiant point, and divide the product by the 

 difference between twice this focal distance and the radius, 

 when the quotient will be the focal distance of reflected rays. 

 Thus, if the radius of the mirror is six inches and the dis- 

 tance of the lamp flame twelve inches, the focal distance of 

 the reflected rays will be four inches (6 x 12-r-12 x 2 — 6 = 4). 

 If the lamp is brought nearer to the mirror, the focal distance 

 of the reflected rays is increased, and vice versd. It will thus 

 be seen how necessary it is to accurately adjust the lamp and 

 the mirror in order to obtain the best results. This also shows 

 how essential it is that the mirror should be mounted, so that 

 it can be slid up and down upon the bar which carries it. 



In Fig. 3, Plate XLIX., is represented the course of a pencil 

 of parallel rays falling upon a concave mirror and reflected to 

 the object. A and B are the extreme rays of the pencil, c the 

 mirror, and the object. 



A great amount of microscopical work can be done by 

 means of the mirror alone, but when object-glasses of high 

 power have to be used, a stronger concentration of the light 

 upon the object becomes necessary. This is accomplished by 

 means of what is called a suh-stage condenser. This is a com- 

 bination of lenses fitted under the stage of the microscope, 

 which condenses the light reflected from the mirror more 

 strongly upon the object. In this case the plane side of the 

 mirror should always be employed. For such a condenser a 

 sub-stage to the microscope is necessary, and it must be so 

 fitted that it can be moved up and down, so that it can be 

 made to approach nearer to or recede from the stage, as may 

 be found necessary. 



Sub-stage condensers are always provided with some means 

 of controlling the amount of light transmitted by them. This 

 is accomplished either by a turn-out arm with a cell for the 

 reception of small discs of metal pierced with holes of dif- 

 ferent sizes, or by an iris-diaphragm, consisting of a series of 

 thin metal plates actuated by a lever which contracts or in- 

 creases the size of the aperture for the transmission of light. 



