THE SCIENCE OF MICROSCOPY. ^^^ 



support his own views!) is apt to forget that the explanation 

 which presents itself is commonly biassed by his previous state of 

 expectancy (belief or disbelief) regarding the nature of the object 

 which he is examining, and is rarely based in the first instance 

 upon any appeal to optical science, as would seem but natural and 

 reasonable in the interpretation of an optical image. 



And this brings us to the last point that can be noticed in the 

 present paper, namely, the physiological differences of visional 

 faculty in different individuals. 



When we find the same object seen at the same time and with 

 the same illumination under the same microscope, differently 

 described by different observers, impressions of light, color, form, 

 size, surface, perspective, &zc., operating with m.arked difference of 

 effect upon the several organs of sight of these observers, we 

 cannot but recognise a fundamental inequality of individual 

 endowment of natural powers. And allowing for all that education 

 of the sight can effect in equalising this difference ; allowing too 

 for the foresight, so to speak, with which the special study of 

 optical effects seen in the microscope image sharpens the sense, 

 there remains a certain inequality, whether of mechanical sight or 

 mental insight, which must be placed to credit or debit (not 

 discredit !) of each observer. It is sometimes rather loosely 

 expressed by the phase "'personal equation." 



Something may be attributed to individual aptness or want of 

 aptness in adapting the seeing faculty to the altered circum^stances 

 of vision through the microscope. In the normal exercise of vision 

 things are not mentally put together by succession of optical 

 sections seen one after the other, as in focussing through a 

 microscopic object. On the contrary, the retina receives and 

 perceives at the same time a number of objects imaged in their full 

 perspective or stereometric form, together with the relations of 

 each group of objects to the other. Then, again, we commonly 

 see all objects by reflected light, transparencies being quite 

 exceptional, while most objects of scientific investigation in which 



