ACHROMATIC CONDENSER. xvii 



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. It may be remarked that the higher object-glasses consist of 

 three combinations of a doubly convex and a plano-concave lens cemented together so as to 

 form apparently a single plano-convex lens ; the outermost and smallest combination some- 

 times consists even of three lenses. When the achromatic condenser is used, the flat surface 

 of the mirror should form the reflecting surface, and care should also be taken that the axis 

 of the condenser coincide with that of the object-glass. To ensure this, a small circular disk 

 of brass having a small 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. As 

 soon as this has been effected, the brass disk must be removed. 



In using the stop in the condenser, its position must also be accurately central. This 

 may be determined by placing a piece of tracing paper upon the upper surface of a slide 

 laid upon the stage, and, by the aid of the axial adjustment, bringing the condensing lenses 

 as near as possible to the lower surface of the slide. On suitably inclining the mirror, a 

 minute black disk surrounded by a luminous ring will be seen with the naked eye upon the 

 paper, and on depressing the condensing lenses, a brilliant luminous spot will occupy the po- 

 sition of the black disk. On next removing the tracing paper, and approximating the object- 

 glass connected with the body to the condensing lenses, and suitably inclining the mirror, the 

 black disk will be seen on looking through the microscope with the eyepiece removed. The 

 disk should then be made to occupy the centre of the field as nearly as possible. It cannot, 

 however, be accurately centred in this way. But this may be easily done by inserting the 

 eyepiece and approximating the object-glass to the condensing lenses until the black disk 

 becomes again visible on looking through the microscope. On then suitably inclining the 

 mirror and arranging the lateral adjustments^of the condenser, the disk may be placed exactly 

 centrally. If the black spot be visible in the examination of an object, elevating or depress- 

 ing the condensing lenses will cause it to disappear. 



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

 behind the first pair of lenses of the condenser, and screwed into the top of the condenser in 

 the place of the ordinary diaphragm. Neither of these kinds of stop equal in convenience 

 the new condenser as constructed by Ross, and Smith and Beck ; because with the latter, 

 the number of rays transmitted or intercepted, and the degree of their obliquity, can be 

 varied by the simple rotation of the diaphragm. Exactly the same effects can however be 

 produced with a little more trouble. 



The central stop is generally used when objects are examined with the higher powers. 

 The power used in the condenser will vary greatly according to the kind of object under 

 examination. If a considerable amount of light be required without obliquity of the rays, 

 the condensing power should be lower than that of the object-glass. If great obliquity of 

 the rays be required, the higher the power of the condensing lenses, and the larger their 

 angular aperture, the better. When the achromatic condenser is properly arranged in regard 

 to centring, and the condensing object-glass or set of lenses is properly selected and adjusted, 

 the structure of minute objects is displayed in a manner with which those who regard the 

 condenser as useless must be utterly unacquainted. A very simple and inexpensive achromatic 

 condenser may be easily contrived by any one with ordinary ingenuity. 



Extra Eyepieces. Always one and sometimes two eyepieces are obtained with the micro- 



