APPENDIX. 501 



field of the microscope, adjust the illuminating lens so that the 

 red light is got rid of and the blue light appears ; the detail 

 of the object will now assume a pearly appearance. The stage 

 is then to be moved round, carrying the object circularly, until 

 the illuminating pencil intersects the structure in the proper 

 direction. The excentricity of the object to the image of the 

 aperture is to be varied also until the best effect is produced, 

 the larger apertures of objectives permitting the greater 

 amount of this excentricity. To recapitulate, the success in 

 bringing out the peculiar effects of this illumination depends 

 upon employing the most appropriate aperture in the diaphragm 

 plate, producing the image of it by means of the suitable rays, 

 by placing the object at the best distance laterally out of the 

 image of the aperture, and by giving the object such position 

 that the illuminating pencil intersects its structure in the 

 proper direction. 



MESSKS. SMITH & BECK'S IMPROVED LAEGE AND SMALLER 

 ACHROMATIC MICROSCOPES. 



The improvement in the microscopes, represented in plates iii. 

 and iv., consists in the new mode of applying the apparatus 

 under the stage, for the important purpose of illuminating the 

 object; that of the smaller instrument is shown in plate xii. 



The difference in construction is simply a short cylindrical 

 tube, which is capable of being moved up and down by rack 

 and pinion, S ; the tube, K, being mounted on the same piece 

 of metal, U U, and by similar fittings as the body of the 

 microscope ; it is, therefore, exactly centrical with it. 



The advantages are, that many of the fittings are simpler 

 and more exact; combinations of apparatus chiefly in connec- 

 tion with polarized light, and never before effected, are easily 

 made; and from the facility of keeping the size, the instrument 

 is not required for future additions of apparatus. 



Besides this, the stages are made thin, to obtain as oblique 

 light as possible, and a revolving plate, T, which is sometimes 

 a convenience, is applied at the base of the pillars. The dia- 

 phragm is shown at L; it is mounted on a short piece of tube, 



