OF THE MICROSCOPE. 171 



there should be need for greater pressure than that which the 

 spring of the wire affords, this can be obtained by sliding a 

 small brass ring on to the clip. 



Various other forms of spring clip have been invented, but 

 none that we consider more simple, or that we like better than 

 the above, which has this great merit, that any one can make it 

 for himself out of materials that may be obtained at any hard- 

 ware store. It must be borne in mind, however, that all clips 

 constructed upon this plan are apt to cause a slight displace- 

 ment of the object, from the fact that the movement of the 

 point is not quite perpendicular. With delicate objects this is 

 a matter of importance. The only remedy is to use the end 

 pressure of a rod moving in fixed guides. 



CEMENTS AND VAKNISHES. 



A supply of carefully selected cements and varnishes is indis- 

 pensable to the microscopist, and it is also well that he should 

 understand the nature and properties of the materials used, 

 otherwise he will be liable to make gross blunders. Thus, of 

 the different articles in use, some are easily mixed with each 

 other, while others separate as soon as left to themselves; some 

 dry in one way and some in another. It would require a vol- 

 ume to detail the properties of the different substances which 

 enter into the composition of the cements used by the micro- 

 scopist. We have space for only the following hints, which, 

 however, we hope will prove useful. 



Cements become hard in three different ways cooling, evap- 

 oration and oxidation. Shellac, sealing wax, electrical cement, 

 etc., when melted by heat, furnish examples of the first pro- 

 cess. Shellac and sealing-wax dissolved in alcohol, and asphalt 

 and damar dissolved in turpentine, dry by the second process 

 the solvents evaporating and leaving behind the material which 

 they had dissolved. Drying oil in all its forms, such as gold 

 size, paint, etc. , becomes hard by oxidation not, as is gener- 

 ally supposed, by evaporation. 



In the case of varnishes which dry by the evaporation of 

 some of their constituents, it is obvious that if a fresh layer 



