^^6 THE SCIENCE OF MICROSCOPY. 



high powers are required^ are seen as transparencies. In this last 

 case the brightness with which any given point in the image is 

 ilhiminated depends on the light proceeding from the corresponding 

 point of the object, and its maximum is reached when the whole 

 aperture of the objective is occupied by the light cone proceeding 

 from it. Conseque7itly the intensity of light and shadow varies loith 

 the angular aperture of the oljectivc,'^ and is not the same for 

 different magnifying powers. But as we are not accustomed to 

 see things with the naked eye under such variation of light and 

 shadow^t we have to learn to see in a new way when looking 

 through the microscope, and to accomplish this, practice is before 

 all necessary. It can however scarcely be denied that such practice 

 may be assisted and lightened by correct theory and mathematical 

 calculation, or that microscopical observation is entitled to be 

 considered a science '^ sui generis.''' 



Finally, if the settlement of a correct or incorrect observation 

 depended on the personal qualification of two observers, each of 

 whom trusted solely and implicitly in his own natural power of 

 sight, (however unequal the two might be in actual performance) 

 the opinion of the quicker sighted observer mJght be so influenced 

 by mental prepossession, disqualifying him from seeing things as 

 they are really presented to his eye, that the only question between 

 them would be whether imperfect, or mentally biassed sight, 

 proved most misleading ! Such a case has happened, and will 

 happen again and again. Is there no appeal to optical certainty, or 

 to the arbitration of pure science in the settlement of such 

 questions ? 



* Assuming the special function of the diaphragm in determining the size of 

 iUiiminating pencil to be regulated in accordance with the aperture of the 

 o^bjective. 



t Except within certain narrow hmits, namely, the largest and smallest 

 area of pupillary opening. 



