ACHEOMATIC CONDEXSEK. xix 



moveable in an arm which is inserted into some part of the stand of the microscope. Their 

 purpose is equally well effected by a slide beneath which a piece of black velvet has been 

 fastened by marine glue. 



Achromatic Condenser. This consists of an achromatic object-glass, or set of lenses, placed 

 in an inverted position beneath the stage, usually in a "secondary" stage, and moveable 

 in all directions in its own plane and in the direction of its axis. It serves to condense the 

 light reflected by the mirror to a focus upon the object, and to exclude all extraneous light. 

 It is essential in examining minute objects with high powers ; in fact, the structure of many 

 objects cannot be made out without it. In the excellent Gillett's condenser, a rotating dia- 

 phragm is placed behind the back glass of the combination forming the condensing object- 

 glass, perforated with a series of apertures of various sizes, some of them being circular, 

 whilst others are annular the former diminishing or increasing the cone or pencil of rays 

 reflected from the mirror by excluding the lateral rays, the latter admitting only the lateral 

 rays, the central ones being intercepted by the portion of the diaphragm within the ring, 

 so that the angular inclination of the transmitted rays may be increased or diminished at 

 will. In its most improved form it consists of two concentric revolving diaphragms, with 

 central stops, by which the relative sizes of the apertures and stops can be varied ; and its 

 angle of aperture is 170 (Powell). In the latest form, it represents a " swinging " substage. 

 The markings upon many of the Diatomacese can only be made out when examined by 

 oblique light, as procured by intercepting the central rays, which effect is produced by this 

 modified achromatic condenser. The same effect may be produced to some extent in one 

 of the achromatic condensers of the old form, provided the compound lenses of which the 

 object-glass in the condenser consists are separable, by pasting or temporarily placing a 

 circular disk or " stop " of black paper exactly upon the centre of the plane face of the 

 innermost combination. The diameter of the disk should amount to about two thirds of 

 that of the surface of the combination to which it is applied. The combinations are then 

 fitted together as they were at first. This stop intercepts the central rays, thus diminishing 

 the amount of light transmitted; but this difficulty is easily got over. When the 

 achromatic condenser is used, the flat surface of the mirror should form the reflecting 

 surface, and care skould also be taken that the axis of the condenser coincides with that 

 of the object-glass. To ensure this, a small cap of brass having a minute circular aperture 

 in its centre should be fitted to the lower part of the tube in which the condensing lenses 

 are situated. When the object-glass is properly adjusted with regard to the condensing 

 lenses, the field of the microscope will appear black, excepting at a minute luminous 

 spot. This spot must be made to occupy the centre of the field by moving the laterally 

 adjusting screws of the condenser, or the body of the microscope j as soon as this has 

 been effected, the brass cap must be removed. Or Ross's centering-glass may be used : this 

 consists of a tubular eyepiece cap, in which are two plano-convex lenses so adjusted that 

 the image of the aperture in the object-glass, and the images of the apertures of the lenses 

 and diaphragms of the condenser, may all be seen in focus at the same time, and their 

 centricity or excentricity determined. 



The focus of the condenser must be made to fall upon the object, which can be effected 

 by raising or depressing the condenser until the window-bars by day, or the lamp-flame 

 by night, are brought into focus ; but if the image of these interferes with the view of an 

 object, the condenser must be lowered to displace them. 



The paper-stop may be very advantageously replaced by a blackened metallic stop placed 



