498 APPENDIX. 



compound body to the right hand until it is stopped; place 

 the plane side of the mirror so as to reflect light up the 

 condenser, and lay a piece of clean white paper upon the 

 mirror; then screw an object-glass upon the microscope 

 (one of the lower powers at first), and apply the A eye- 

 piece to the other end of the tube, and, having placed an 

 object on the stage, move the tube down by the rack bar of the 

 microscope until vision of some part of the object-shde is ob- 

 tained; then having removed the object, also the cap from the 

 eye-piece, apply the examining glass in the place of the cap, and 

 adjust this glass until the images of the diaphragms of the object- 

 glass and illuminating lens are distinctly seen. If the illumina- 

 tor be now moved by means of the side screws of the secondary 

 stage, while looking through the examining eye-glass, the cen- 

 tral position of the illuminator under the compound body, in 

 this direction, may be determined and finally adjusted by 

 means of the side screw, c, of the condenser; the central 

 adjustment in the direction at right angles to the above, is 

 obtained by turning the milled head, d, in front of the under- 

 neath stage, which acts by moving the illuminator to and from 

 the observer until the images of the diaphragms are concen- 

 tric, also the white disc formed by the paper on the mirror. 

 Having removed the paper from the mirror, the light of the 

 sky or lamp may be directed up the condenser, and the field 

 of the microscope illuminated ; then replace the object, and 

 obtain distinct vision of it by the adjustment of the com- 

 pound body; while this state of the apparatus remains, the mir- 

 ror should be so inclined, that the image of some intercepting 

 object, as a tree or house-top, should be brought into the field, 

 and, though not distinct, may be recognised by its partially 

 darkening the field of the microscope; then distinct vision of it 

 must be obtained by the rack slide, which moves the secondary 

 stage to and from the object-stage, and, the adjustment of the 

 compound body remaining as it was, the microscopic object 

 wiU be seen distinctly at the same time with the tree or house- 

 top. The mirror may now be turned so as wholly to reflect the 

 light of the sky, and the image of fleeting clouds will be seen 

 as they pass. The brass disc, with the various apertures, may 

 now be moved until that quaUty of illumination is obtained. 



