61 G HISTOLOGY. 



f'ul to Jiave the image thrown on paper by means of a camera-lucida, or small 

 prism, which can be easily attached to the microscope. A polarizing apparatus is 

 a useful appendage. In the apparatus sent along with microscopes will be found 

 a compressorium, for the purpose of applying pressure to objects whilst they are 

 under examination, troughs for holding such plants as Chara, which are to be 

 seen in water, and various instruments for the dissection and examination both 

 of animal and vegetable structures. In testing the power of the instruments, 

 certain minute objects are used, such as the scales of Podura plumbea (common 

 Springtail), of Lepisma saccharina, of Hipparchia Janira (common Meadow 

 Butterfly), the hair of Dermestes, and Muscular fibrilla}.* Certain markings 

 occur in these test-objects which can only be seen by good microscopes. 



1198. In viewing objects under the microscope, they must be placed on 

 slips or slides- of glass, which should be of a uniform size, not less than three 

 inches by one ; and they should be covered with pieces of very thin glass, ^ 

 to T o f an inch thick, and about J of an inch square. The slides ought to be 

 made of thin plate-glass, and the covers of very thin crown or plate-glass. In 

 examining recent vegetable structures, it is best to moisten them with water. 

 When the parts are dry, thin sections may be made either by means of slicing 

 instruments, or by a sharp knife. Many dry objects are well seen when immersed 

 in Canada balsam. To preserve objects in a moistened state, the substances 

 used are alcohol, solution of salt, alum and corrosive sublimate, water contain- 

 ing a small quantity of creosote (5 grains to the ounce), and glycerine. The 

 objects, in such instances, are placed in shallow glass cells, or they are laid on 

 the slides and covered with thin glass, which is cemented by means of japanner's 

 gold size, or black japan varnish. 



1199. Histology. The study of the microspic structure of organized bodies 

 is termed Histology or Histiology (larog, or imtov, a web, or tissue). In the prose- 

 cution of it, it is necessary to have good microscopes, such as those constructed 

 by Powell, Ross, Smith and Beck in London, Oberhaiise & Chevallier in Paris, 

 Ploesel in Yienna, Schiek in Berlin, or Frauenhb'fer in Munich. It is not enough, 

 however, that the instrument should be good ; the student must also know the 

 mode of using it, and of manipulating. Hence the importance of courses of in- 

 struction in this department, such as those given at various medical schools. 

 Constant practice is required in order to avoid sources of fallacy and incorrect 

 observations. The instrument must be steady; all dust must be carefully 

 avoided, the lenses and glass-slides being well cleaned with such materials as 

 well-washed linen, which will not leave any adherent particles. The illumina- 

 tion ought to be good. The best light is that derived from a white cloud. When 

 the sun's light cannot be used, then a sperm-oil lamp with a shade may be 

 employed, the light being concentrated by a condensing lens. In viewing opaque 

 objects, an additional supply of light is obtained by having a concave silvered 

 speculum, called a Lieberkulm, attached to the object-glass. The mirror of the 

 microscope ought to be placed near the stage, in order that the rays may reach 

 the object before crossing. Certain parts of objects may be rendered more vis- 

 ible by the use of reagents. Thus, iodine gives a blue colour to starch, and 

 usually a brown colour to the cell-nucleus and to the cell-membrane ; while sul- 

 phuric, nitric, and acetic acids, and ether, act by removing some matters which 

 obscure vision in particular instances. 



1200. Dr. Allen Thomson remarks, that, in conducting microscopic observa- 



* For Drawings of these Fibrilla;, see Paper by Mr. W. Murray Dobie, in the Annals of Natural 

 History. February, 1849. 



