724 THE MICROSCOPE. 



solatory reflection which accompanies the difficult task of 

 microscopic investigation is the unanimity which bow 

 reigns among observers on so vast a body of observations. 

 If we read in physiological works of the yolk cells and 

 coloured oil globules of the yolk, and the beautiful 

 function of assimilation which has been attributed to 

 them, they exist but in the imagination of the authors 

 who have regarded the one as cells simply because they 

 are round, and the other as consisting of fat because they 

 are highly refractive, — such errors of interpretation do 

 not discredit it any more than the mis-interpretations, 

 which have helped to make Ehrenberg's name at once 

 famous and suspicious, alter the facts which he saw, and 

 could not rightly interpret. In truth, the eye is only 

 a preliminary instrument in science. What we see has to 

 be interpreted ; and, as it is very difficult to confine our- 

 selves to pure observation unmixed by hyiDothetical inter- 

 pretation, we need many collateral confirmations." 



The principal physical characters to be regarded in micro- 

 scopic examinations may be summed up as follows : — 



1. Shape. — Accurate observation of the shape of bodies 

 is very necessary, as many are distinguished by this phy- 

 sical property. Thus the human blood-globules present 

 a round biconcave disc, and are in this respect different 

 from the oval corpuscles of birds, reptiles, and fishes. 

 The distinction between round and globular is very requi- 

 site. Human blood corpuscles are round and flat ; but 

 they become globular on the addition of water. Minute 

 structures seen under the microscope may also be likened 

 to the shape of well-known objects, such as that of a pear, 

 balloon, kidney, heart, &c. 



2. Colour. — The colour of structures varies greatly, and 

 often differs under the microscope from what was pre- 

 viously conceived regarding them. Thus the coloured 

 corpuscles of the blood, though commonly called red, are, 

 in fact, yellow. Many objects present different colours, 

 according to the mode of illumination ; that is, as the light, 

 is reflected from or transmitted through their substance, as 

 in the case of certain scales of insects, feathers of birds, &c. 

 Colour is often produced, modified, or lost, by re-agents ; 

 as when iodine comes in contact with starch-granules, when 



