116 THE MICROSCOPIST. 



Pontia Brassica, or Common Callage Butterfly. The pale, 

 slender, double-headed feathers, having brush-like appendages 

 at their insertion, are good test objects. The specimens which 

 are easily resolved are short, broad, and more opaque. The 

 striae are longitudinal, and with a power of 500 diameters 

 appear to be composed of rows of little squares or beads. 



Podura plumbea. or Lead-Colored Springtail. The body 

 and legs of this tiny creature are covered with scales of great 

 delicacy. The surface of each, under a power of 500 diameters, 

 appears covered with numbers of delicate wedge-shaped dots 

 or scales, .arranged so as to form both longitudinal and trans- 

 verse wavy markings. A very small scale is a good test of the 

 defining power of a one-twelfth or one-sixteenth inch object- 

 glass. The small scales may easily be rubbed off the scale to 

 be examined, unless great care be taken in mounting, &c., and, 

 of course, it will be useless as a test object. 



SHELLS OF INFUSORIA. Several delicate species serve as 

 test objects. The so-called longitudinal and transverse striae 

 are resolved by superior instruments into dots or bead-like pro- 

 jections from the surface. The Navicula hippocampusj JV". 

 angulata, N. Spencerii, &c., have been recommended as tests. 

 A species marked Navicula attenuata, is a good object, re- 

 quiring delicate illumination under a high power, in order to 

 show the longitudinal striae or dots. Several kinds of Tripoli 

 may also be used for the purpose. 



For examining the striae on infusorial shells it is often very 

 necessary to have an oblique illumination. 



As it is always a tedious matter with the use of a high power 

 to find a minute object on the slide under the stage, it will be 

 most convenient to bring it first into the centre of the field by 

 the use of a lower power, and afterwards substitute the high 

 power object-glass. 



ARTIFICIAL OBJECTS. M. Robert, a Konigsberg optician, 



