72 THE MICROSCOPE. 



a steady hand and a well-made animalcule cage, such as we have pre- 

 viously described, will answer the purpose sufficiently. 



Smith and Beck's troughs for chara and polyps, a sectional view of 

 which is shown at fig. 53, are made of three pieces of glass, the bottom 

 being a thick strip, and the front a of thinner glass than the back b ; 

 the whole is cemented together with Jeffery's marine-glue. The method 

 adopted for confining objects near to the front-glass varies according 

 to circumstances. One of the most convenient plans is to place in the 

 trough a piece of glass that will stand across it diagonally, as at c ; 

 then if the object be heavier than water, it will sink, until stopped by 

 this plate of glass. At other times, when used to view chara, the dia- 

 gonal plate may be made to press it close to the front by means of 

 thin strips of glass, a wedge of glass or cork, or even a folded spring. 

 When using the trough, it is necessary that the microscope should be 

 in a position nearly horizontal. A useful trough, described by Mr. 

 Varley, in the Transactions of the Society of Arts, consists of a bottom- 

 plate of stout glass, upon which is cemented with pitch and bees'-wax 

 a thin one for the top ; slips of glass between it and the bottom-plate 

 forming the sides. The top-plate is not so broad as the bottom, in 

 order that a slip of chara may be more readily placed in the trough, 

 as it can be first laid upon the bottom-plate, and then gradually be slid 

 down between it arid the other. In order to render the trough more 

 manageable, it can be cemented to a larger bottom-plate by Canada 

 balsam; but it will be found far more advantageous if the bottom- 

 plate itself be large and broad, and the cover cemented to that, and not 

 to another plate, as two extra surfaces will then be dispensed with. 



Dissecting Knives, &c. Knives and needles of various kinds and 

 sizes are required for microscopic dissection : the best for the purpose 

 are represented in fig. 54, being, in fact, the very delicately made 

 knives used by surgeons in operations upon the eye. Dissecting needles 

 may be either straight or curved. They may be fixed, or made to take 

 in and out of their handles. Three of the most convenient are shown 

 in fig. 55 ; which are made expressly by Mr. Weedon for the use of 

 the microscopist. 



In the preparation of objects, no mieroscopist was ever more suc- 

 cessful than Swammerdam : His chief art seems to have been in con- 

 structing very fine scissors, and giving them an extreme sharpness ; 

 these he made use of to cut very minute objects, because they dissected 

 them equally, whereas knives, if ever so fine and sharp, are apt to dis- 

 order delicate substances. His knives, lancets, and styles were so fine, 

 that he could not see to sharpen them without a magnifying glass. 



