INFUSORIAL ANIMALCULES. 45 



reference has been had to a ten inch standard, and the 

 enumeration of powers has been in diameters, or what are 

 sometimes termed linear-, thus, what I compute at 100 is 

 often spoken of as 10,000, that being the superficial 

 measurement; ample reasons for the adoption of linear 

 measure, and for that standard, are given in my works on 

 this subject. 



In demonstrating minute portions of the structures of 

 Infusoria, a power of 800 diameters will sometimes be 

 requisite, unless the sight be exceedingly good. I have 

 invariably observed that aged persons require greater 

 assistance, in this respect, than young ones. Notwith- 

 standing this, it will be impossible to arrive at an accurate 

 knowledge of the creature you may be studying, even 

 with a power of 800, unless it has been previously 

 examined under a lower one, so that the relations of 

 its several parts may be first clearly understood. When- 

 ever the object in view is merely that of instructive 

 amusement, a power of 250 diameters will be amply 

 sufficient ; that power can be managed with ease, and 

 does not fatigue the observer. The greater number of 

 Dr. E.'s discoveries were effected under a power of 380. 

 I am not aware whether he has mentioned in any of his 

 works the siderial focal length of his object-glasses, or 

 the standard of sight. The set which Dr. E. speaks most 

 in praise of is similar to one which I possess, and which 

 has a focus ofl-7th of an inch. He considers that with 

 " a good achromatic microscope and a lamp, our observa- 

 tions may be carried on at night as well as in the day, 

 which, by some, may be esteemed as an additional 

 recommendation.' 3 



