25 



object magnified by a certain number of diameters it is easy to esti- 

 mate the size of the original object by dividing the magnified or 

 reproduced image by the number of diameters. 



Microscopical Technique. 



For the preparation of objects for examination or for their pre- 

 servation as permanent specimens the following notes from practical 

 experience will, I trust, be of service. 



The necessary apparatus should consist of — a lens suitably 

 mounted for the examination and dissection of the various tissues ; 

 this need not be of an elaborate nature. Most tissues are dissected 

 under water, and a cork slip backed with lead will be found con- 

 venient for attaching the object. Scalpels of various sizes ; scissors, 

 fine-pointed and curved ; needles with flattened points and cutting 

 edges, in light handles ; glass dishes or watch-glasses with flat 

 bottoms for dissecting and staining purposes ; test-tubes, wide- 

 mouth stoppered bottles, a washing bottle, dipping tubes and pipettes ; 

 thin glass strips, 3 inches by 1 inch, with ground edges ; one ounce 

 each thin glass circles or cover-glasses ; these are obtainable at the 

 opticians, and are of varying degrees of thinness, known by the 

 numbers 1, 2, and 3. No. 2 (medium) are the most suitable for ordinary 

 work, but for very thin sections or bacteria No. 1 ("003 to "006 mm. 

 thick) should be used so as to admit of the use of immersion powers 

 which have to be brought almost in contact with the cover-glass. It 

 is well to keep a stock of these having the following diameters : 

 f , f and -§■ inch. A few dozen slips with excavated cells, for mounting 

 objects without pressure, should also be kept. For bacteriological 

 work it is necessary that the slips and covers are scrupulously clean, 

 and I recommend soaking them in strong sulphuric acid and tho- 

 rough washing in water. 



List of chemicals, etc., required. — Acids: acetic, arsenious, carbolic, 

 chromic, hydrochloric, nitric, osmic 1 per cent., picric and sulphuric; 

 alcohol absolute, benzole, chloroform, Canada balsam, dammar, ether 

 meth., glycerine liq. potassae, oils of cedarwood, cloves, and turpen- 

 tine, nitrate of silver, chloride of gold, perchloride of mercury ; 

 and the following staining reagents : aniline oil, carmine, eosin, 

 fuchsin, gentian violet, hematoxylin, methylene blue, bismarck 

 brown, etc. 



In the majority of cases the ordinary method of preparation of 

 objects will be satisfactory — /. e. treatment with liquor potassae 

 (especially if the object be chitinous), dehydration with alcohol, 

 clearing with oil of cloves or turpentine, and mounting in Canada 

 balsam. This procedure applies particularly to insect preparations. 

 For some objects, however, Canada balsam is unsuitable, more 

 especially in mounting pediculi, entomostraca, and other soft and 

 transparent structures. For these, glycerine in the form of Farrant's 



