58 PKACTICAL TREATISE ON THE MICROSCOPE. 



zontally upon a steel pin at the top of the square stem. The 

 stem is lowered and raised by a pinion with a large milled 

 head, I, two inches in diameter, by which tolerably fine adjust- 

 ments may be made, but finer still may be effected by the 

 lever, o, which fits into a series of holes drilled in the circum- 

 ference of the same milled head, /. The whole of this adjusting 

 apparatus is attached to a plate of brass, jj, and is made to 

 shde into another plate, i i, fixed to the back of the case by 

 screws. When not in use, the entire apparatus on the top of 

 the case may be removed and placed in a box or drawer in its 

 interior. When transparent objects are being dissected, the 

 screen, q, made of black cloth, may be attached in front of the 

 stage by two brass pins, pp; this screen or curtain has a two- 

 fold use, the one to intercept all extraneous light save that 

 reflected from the mirror below, the other to keep the light of 

 the lamp or candle employed in the illumination from the eyes 

 of the observer. The pins, p p, are bent a little forwards, 

 that the curtain may not be in the way of the head. The 

 microscope is thus arranged for the dissection of transparent 

 bodies, such as the vessels or other tissues of plants, for which 

 the inventor, Mr. Slack, was so celebrated ,■ but when opaque 

 objects are under examination, the condensing lens must be 

 employed ; this may either be fixed on a separate stand, or to 

 some part of the top of the case. An improvement has been 

 made by Mr. Goadby in this dissecting microscope of Mr. 

 Slack : he places the stem for the adjustment, in the interior 

 of the case, and the milled head only is allowed to project on 

 the outside ; this can be put on or taken off at will, as the 

 end of the pinion is made square to receive it. The case is 

 on rather a larger scale than Mr. Slack's, but in shape is 

 precisely similar. As most of Mr. Goadby's dissections are 

 carried on under water, square tin troughs are used for the 

 purpose, each of which has a circular ring fastened to the 

 bottom, to fit into the aperture, g, of the stage, and by this 

 means they are prevented from shifting their position. 



A very useful single microscope is that made by Mr. Ross, 

 and described by him in the Penny Cyclopedia, article, " Micro- 

 scope." It is represented by fig. 37, and consists of a brass 



