OF MICROSCOPIC OBJECTS. 15 



Rings of cardboard, &c., have been rejected by persons of great 

 experience, because they are of such a nature that dampness can 

 penetrate them. This fault can be almost, if not totally, removed 

 by immersing them in some strong varnish, such as the asphalt 

 varnish hereinafter mentioned ; but they must be left long enough 

 when affixed to the glass slide to become perfectly dry, and this 

 will require a much longer time than at first would be supposed. 



There has, however, been lately brought out what is termed the 

 ivory cell. This is a ring of ivory-like substance, which may be 

 easily and firmly fixed to the glass slide by any of the commonly- 

 used cements, and so forms a beautiful cell for any dry objects. 

 They are made of different sizes, and are not expensive. 



Sometimes slides are used which are made by taking a thin 

 slip of wood of the usual size (3 in. by 1 in.), in the centre of 

 which is cut a circular hole large enough to receive the object. A 

 piece of thin glass is fixed underneath the slide, forming a cell for 

 the object, which may then be covered and finished like an ordi- 

 nary slide. This has the advantage of serving for transparent 

 objects for which the before-mentioned wooden slides are unsuita- 

 ble. A slight modification of this plan is often used where the 

 thickness of the objects is inconsiderable, especially with some of 

 the Diatomaceae, often termed "test-objects." The wooden slide 

 is cut with the central opening as above, and two pieces of thin 

 glass are laid upon it, betwixt which the diatoms or other objects 

 are placed, and kept in their proper position by a paper cover. 

 This arrangement is a good one, insomuch as the very small por- 

 tion of glass through which the light passes on its way to the mi- 

 croscope from the reflector causes the refraction or interference to 

 be reduced to the lowest point. 



A novice would naturally think the appearance of some of the 

 slides above mentioned very slovenly and unfinished ; but they are 

 often covered with ornamental papers, which may be procured at 

 almost every optician's, at a cost little more than nominal, and of 

 innumerable patterns and colours. How to use these will be de- 

 scribed in another place. 



It is very probable that a beginner would ask his friend what 

 kind of slides he would advise him to use. Almost all those made 

 of wood are liable to warp more or less, even when the two pieces 



