DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 107 



into which is fitted by a bayonet-joint the spring safety 

 slider- holder, fig. 20, or the finely-adjusting one de- 

 scribed in the Micrographia, page 217. 



The utility of these safety-slider-holders is, that if by 

 any accident the object-glass should come in contact 

 with the slider containing the object when it is placed 

 between the plates, the object-glass will not be damaged; 

 for the spring which acts upon the lower plate will yield. 

 I most strongly recommend that a good object-glass of 

 deep power should never be used without this or a 

 similar contrivance. The numbers which are injured or 

 destroyed for want of this precaution are very great. 

 It is often quite impossible to restore them to their pri- 

 mitive excellence, even though the anterior lens of the 

 combination should not be broken. It may be remarked, 

 also, that in half the accidents of this kind which occur, 

 the observer is wholly unconscious of what has happened, 

 his attention being riveted to the object he is engaged 

 with. 



SECT. 4. The Spring -Phial-Holder. This apparatus is 

 designed to afford us all needful assistance towards inves- 

 tigating a certain class of objects without removing them 

 from their natural element fluids. The circulation or 

 cyclosis in plants, the circulation in the tail of a fish, the 

 structure and functions of various marine and fresh- 

 water zoophites, the habits of such entomostraceans and 

 infusoria as attach themselves to the sides of vessels, the 

 development of the young from the ova of creatures 



