Reade, on a New Hemis^jherical Condenser. G3 



la the application of this condenser to the resolution of 

 lined test objects, it will be seen that the principle sought to 

 be carried out is to throw the axis of the pencil of illumi- 

 nating rays in a direction at right angles to the line to be 

 resolved. In all cases where the precise position of such 

 lines is known, a supplementary diaphragm may be cut with 

 the apertures in their correct mutual positions ; but as these 

 position angles greatly vary in diflferent diatoms on the same 

 slide, my friend, Mr. Waterhouse, ingeniously suggests the 

 use of a pair of similar diaphragms overlying each other, and 

 capable of revolution round a common centre. For this 

 purpose the diaphragm next the condenser must be fixed in 

 position, and moveable with the lens, by means of the pinion 

 motion of the sub-stage, while the other is attached to a deep 

 hoop fitted upon the brass tube carrying the lens, so as to be 

 conveniently rotated by the finger and thumb, applied to a 

 narrow milled ring, but suflSciently small to pass through the 

 opening of the second stage, when the condenser is required 

 to be removed for other purposes. To carry out this sug- 

 gestion, place two diaphragms together, and mark out on 

 their circumference the positions of six adjacent apertures; 

 cut out one aperture, pass over two, and cut out the remaining 

 three ; then tui-n them face to face, so that the small stops 

 between the apertures may coincide, and, by the rotation of 

 one diaphragm upon the other, the stop between two aper- 

 tures, or little prisms, as they virtually are, may be made to 

 vary from about 30° to 120°. This will be ample scope for 

 all bilinear, oblique, and rectangular markings. This 

 method of arrangement also admits of the introduction of 

 a third aperture for the P. angulatum, &c., and the whole 

 diaphragm system is thus brought within the least possible 

 compass. 



The lens in its present form is simple, cheap, and easy of 

 adjustment, though of course not free from chromatic aber- 

 ration ; but the proper adjustment of the apertures to the 

 object examined seems to prevent this error from being very 

 apparent, and a pierced diaphragm beneath as well as upon 

 the condenser has advantages in this direction, as well as 

 occasionally in others. The central pencil of about ^th of 

 an inch in diameter, Avhich gains intensity from the con- 

 struction, is itself virtually achromatic, and is also very 

 effective for direct central illumination where obliquity is not 

 required, or would be injurious. 



The angle of aperture of the lens is necessarily small ; and 

 therefore I cannot help thinking, with Mr. Tomkins, that 

 if it were possible by the application of rotating pierced 



VOL. 1. NEW SER. / 



