434 The Eye and the Microscope. 



tions, as the case may be, out of focus. As a general rule, the 

 larger the angle of aperture of the object-glass, and the more 

 oblique the rays it can receive from the surface of the object, 

 the more cloudy will be any parts that are not focussed to a 

 nicety, and hence, for ordinary use, it is well that a quarter- 

 inch power should not exceed about 80 or 90 angular aperture, 

 and a fifth a little more. The impossibility of making any high 

 power work well on uneven objects indicates a practical restric- 

 tion of their use ; and when they must be employed upon things 

 which they can only show in part, no attention should be paid 

 to what is indistinct. This habit is easily formed by concen- 

 trating attention upon that which is seen well ; and if this is 

 accomplished, a fertile source of optical discomfort is removed. 



Let one more hint be given on the avoidance of injury to 

 the eye. Secure steadiness in the instrument, the table, and 

 the place of observation. It is extremely fatiguing, and taxes 

 the brain as well as the eye, to try to make out the details of 

 objects that are fidgetting about. Slides merely require a 

 good instrument, a table which no one shakes, and a room in 

 which no one runs or stamps about ; but live things must be 

 restrained by slight compression, or by a loop of thread, which 

 when pressed down in the live box, forms a sort of cage, the 

 whole of which can be taken in by the power employed. 



It would be easy to prolong these hints on the eye and 

 the microscope ; but if, the student can be set to work in 

 the right manner, he will soon be able to profit by the labours 

 of well-known writers ; and if, during the first three months of 

 his engaging in microscopic pursuits, he will determine never 

 to be satisfied unless his objects are seen as easily and as 

 plainly as the furniture of the room, he will not, in any sub- 

 sequent portion of his career, complain that the employment 

 of the most fascinating of optical instruments has injured his 

 sight. 



