TN^rrRORIAT. ANIMALrtTLES. 



105 



powers of microscopes are spoken of, the standard of siglit used in 

 computing them should be Imo-mi, otherwise veiy eri'oneous ideas will 

 be formed. In all my publications, fi-om 1827 up to the present time, 

 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 of that standard, are given in my works on this 

 subject. 



In demonsti-ating minute portions of the structiu'es 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. Not- 

 withstanding this, it will be impossible to anive at an accurate 

 knowledge of the creature you may be studying, even mth 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. 

 "Whenever 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 Ehrenberg's discoveries were effected under a power of 380. 

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

 sidereal 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 employ, and which has a focus of l-7th of an inch. He considers 

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

 tions maybe carried on at night as well as in the day, which, by some, 

 may be esteemed an additional recommendation." (For further in- 

 fonnation on the illumination of Infusoria, see Part III, Bacillaria.) 

 Section III. — On Micrometers, and the Method of Measuring In- 

 fusoria. — The late Dr. Goring, in the Microgra/phia, has described the 

 method by which, in various ways, a correct admeasurement may be 

 taken of these minute creatui-es, as also Mr. Bauer, in a paper in the 

 same publication. I cannot do better than refer the reader to these 

 authorities, for the fullest infonnation attainable on this subject. A 

 few words, however, may be said on the mode of proceeding, which 

 I have myself adopted, and which, after much practice, has been 



