198 THE MICROSCOPE. 



of the object getting out of focus. 3d. If the tail he not 

 pushed too far into the cell, the vessels at its root are not 

 compressed, and the circulation goes on very freely. 

 4th. The fish may be kept on the stage of the microscope 

 for two or three hours without injury. Mr. Macaulay has 

 suggested, as an improvement, that the three sides of the 

 cell should be made of a single strip of plate-glass, bent to 

 the shape represented in our wood-cut. 



Growing-cells. — Considerable attention has been given 

 to various forms of growing-cells for maintaining a con- 

 tinuous supply of fresh water to objects under constant 

 observation, fur the purpose of sustaining vital growth for 

 a long period. The employment of such cells is strongly 

 commended to microscopists, as there is yet much to 

 be discovered concerning the metamorphoses which some 

 of the lowei microscopic forms of plant and animal life 

 pass through ; a patient investigation will probably show 

 that many which are now classed as distinct species 

 are merely different phases of the same type, which alter- 

 nate in a higher or lower scale of development accord- 

 ing to the varied conditions of temperature and nutrition 

 under which they are grown. 



Professor Smith, of Kenyon College, furnished us with 

 what he has called a growing slide, or trough. It con- 

 sists of two pieces of thinnish glass cemented together; 

 in one corner of the upper cover a small hole is bored, 

 and through this a fresh supply of water is introduced 

 without in any way disturbing the desmid, or living object 

 under inspection. Mr. Beck has contrived an improved 

 form of cell ; but, whilst the first-mentioned may be con- 

 structed for a few pence, the latter cannot be had for a 

 less price than ten shillings. 



Dissecting Knives, <&c. — Knives and needles of various 

 kinds and sizes are required for microscopic dissection ; the 

 best for the purpose are represented in tigs. 124, 125, and 

 126, 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. The most con- 

 venient are shown in fig. 125; those made of Palladium 

 by Mr. Weedon, Hart Street, are very much the best. 



