OF THE OYSTER, MUSSEL, AND COCKLE. 17 



filaments ; the other represents a bundle breaking up into its individual sperma- 

 tozoa. B is a bundle of imperfect sperm cells. 



3. — A is a drawing of a stained section of generative gland containing eggs 

 The dark portions represent the places where the male element was. In this 

 animal it had not entirely disappeared. 

 B are eggs. 



4. — C is an egg after its discharge from the gland, when it is retained between 

 the branchial plates, &c, until it reaches the larval form. 



5. — A B, possessing cilia, by which it moves freely, and is soon puffed out of 

 the parent shell. 



Notes on " Personal Equation," with reference to 



Microscopy. 



By John E. Ingpen. 



(Communicated Nov. 27, 1874.) 



Abstract. 



The subject of Personal Equation is probably less interesting 

 tban important. It is well recognised in astronomy, especially as 

 regards transit observations. But tbe term is also applied by 

 astronomers to all differences by a constant quantity between 

 observers, sbort of actual defects of vision. The subject bas not 

 attracted so much attention among microscopists as it deserves, and 

 it is worthy of consideration now that very delicate observations 

 are being frequently made, and high powers employed ; while many 

 differences of opinion, otherwise inexplicable, may possibly be 

 traced to this source. Mental personal equation is very obscure, 

 though it certainly exists. It may be suggested that many of us 

 see what we wish, or expect, or have been taught to see — par- 

 ticularly in the case of test objects. As an instance may be quoted 

 the difference of opinion as to when certain objects, such as blood 

 discs or Podura scales, are in focus. The interpretation of this 

 and similar problems seems subject as much to an " equation " as 

 to any difference in reasoning.* Closely allied to this is nervous 

 equation, as when observations are affected by the greater or less 



* Dr. Richardson, in an article in the "Monthly Microscopical Journal" for 

 January 1, 1875, p. 21, remarks: — " How few investigators have minds achro- 

 matic enough to enable them to see objective facts without subjective colouring." 



