364 The Microscope. 



cover. The cover therefore pi'otect^ the objective with its drop 

 of immersion fluid, and it protects also the object from the 

 action of that fluid. 



But among all these important functions perhaps the most 

 important is to flatten the drop of medium in which the object 

 is embedded. When this mounting fluid is placed on the object 

 as it lies upon the slip, it acts as a drop of liquid will always 

 act. It would form a spherical drop if the slip itself did not 

 flatten out the lower side ; but as well as it can it follows the 

 law of Nature that commands every free drop to become a 

 sphere, and forms its upper surface into a curve that is that of 

 a hemisphere since its lower surface is flattened. Now this 

 liquid hemisphere has a very unwelcome action on the light 

 that passes through it from the mirror below the stage of the 

 microscope. The action is optically so peculiar and therefore 

 so unwelcome, that the object can scarcely be seen, acquiring a 

 very strange appearance by being puckered and distorted in 

 a way that would lead the microscopist to erroneous opinions 

 as to its character and structure, if there was no way to get rid 

 of this hemispherical drop that is the cause of the mischief. 

 When the drop is flattened, the light will pass through it with- 

 out giving the microscopist these optical troubles, and to flatten 

 it he uses the film of thin glass called the cover, an exceedingly 

 important little contrivance, and one with which the beginner 

 in mounting will have the severest struggles before he conquers 

 it. But it is tractable when once mastered, and the process of 

 getting it into subjection is not very great; it needs only some 

 care and some intelligent attention. In this it is like all things 

 connected with the microscope. At first the whole instrument 

 will be rebellious, and the beginner may be discouraged ; but a 

 little intelligent application of an intelligent mind to a piece of 

 inanimate matter will meet with success, and the instrument will 

 soon become a helpful companion and an entertaining one. 



At first the absence of thickness in cover glasses and the 

 facility with which they break are surprising ; and a broken 

 cover is good only to be thrown away. At the beginning, too, 

 the novice at mounting will break about every alternate cover 

 that he touches, until he thinks that his experience is excep- 

 tional ; but it is not. Everybody passes through this breaking 

 stage. It is of only short duration; the worker soon learns to 



