WITH THE MICROSCOPE. 247 



after the other, to be placed on the end of the tube of the microscope 

 arranged vertically, and thus secure small but very perfect represen- 

 tations for future enlargements, or for being viewed in the stereo- 

 scope. He also speaks highly of the objectives a T immersion; a 

 drop of distilled water being placed on the cover of the slide and 

 the front lens focussed into and through the fluid, by which means 

 the refraction occasioned by the thin cover is avoided. The sur- 

 face of the lens is not to be wiped, but the water blown away. By 

 continental microscopists these lenses, especially those made by 

 M. Hartnach, have been very highly spoken of. Dr. Moitessier also 

 employs an ingenious method for rendering opaque objects with the 

 horizontal microscope and low powers. The object is placed on the 

 stage of a small vertical microscope, and the light thrown on the 

 object by a small plane mirror from above, which receives the solar 

 rays, after having been converged from a larger mirror, by an 

 achromatic lens ; this and the small flat mirror are supported by, and 

 slide on, an upright stem, to meet the necessary adjustments. The 

 objective is attached to the end of the microscope tube at right angles, 

 a prism with total internal reflection being fixed at the junction. 



The focus is obtained by the rackwork acting on the small stage. 

 For very low powers or securing the enlargement of only a few 

 diameters, as in injected specimens and entire insects, a small 

 portrait combination is- attached to the microscope tube, and the 

 prism placed in front of it at right angles. 



331. Arrangement of Drs. Abcrcrombie and Wilson. Drs, 

 Abercrombie and Wilson r of Cheltenham, have met with great 

 success with artificial illumination. These gentlemen use a 

 blackened base-board 8 feet in length; the focussing box of an 

 ordinary camera with its focussing screen, the microscope and 

 illuminating apparatus being all kept in a straight line by side 

 strips of wood. The microscope is moveable on a sliding board 

 and can be clamped at any distance, or the camera box and micro- 

 scope made to approach or recede from each other singly or together. 

 A couple of strips of blackened wood are attached to the eye-piece 

 end of the tube of the microscope, and brought slightly diverging to 

 the top of the camera. The whole of this part being covered with 

 black velvet, pile inwards, and well secured from outside light at all 

 parts, especially round the tube of the instrument. The base-board 

 can be set on any steady table or support. The focussing screen is 

 of glass covered with collodion, sensitised and covered with a 

 solution of tannin. The draw tube of the microscope, if any, is re- 

 moved and the tube lined with black velvet. The correction for the 

 want of concordance of the actinic and visual focus is effected by 



