TEST OBJECTS. 479 



Method of Examining Test Objects. — For the purpose of 

 examining these most delicate of all structures, considerable 

 care and sldU are required, the more so if two object-glasses 

 of equal power are to be tried one against the other. The 

 usual modes of illuminating transparent objects have already 

 been given at page 186, where also will be found the descrip- 

 tion of the different kinds of apparatus which are placed beneath 

 the stage, in order to increase the brightness, and cut off the 

 outer rays of the illuminating pencil. The objects for exami- 

 nation should be perfect specimens, and mounted either in the 

 manner represented by figs. 210-11-12-13, or on a slide of the 

 usual size, and covered with the thinnest films of glass; some 

 of the most opaque specimens may be put up in balsam, but 

 the majority are far better seen when mounted in the dry 

 way between pieces of thin glass. Day-light will be found to 

 be the best for all examinations, and the light reflected from 

 a white cloud, save that of the sun, the brightest that can be 

 obtained. The microscope having been placed on a firm table, 

 in a suitable situation to get a good light on the min-or, and 

 everything ready, the object-glass, if of high power, must next 

 be corrected for the thickness of the glass cover; as the 

 method of doing this was not described at page 175, it will 

 be proper to mention it here. 



Method of Using the Adjusting Object-glass. — As the high 

 powers of Messrs. Powell and Eoss have the same kind of 

 adjustment, the following directions, drawn up by Mr. Ross, will 

 answer for both; but those of Mr. Smith, being of a different 

 construction, wiU require a separate mention: — 



" When an achromatic object-glass for a microscope has its 

 aberrations corrected for viewing an uncovered object, the cor- 

 rection will be nearly the same, whether the object is seen by 

 the light reflected from its surface as an opaque, or by its inter- 

 cepting transmitted light as a transparent one, if these objects 

 are properly prepared and illuminated. But if it be necessary 

 to cover the object with glass or talc, or to immerse it in a fluid, 

 the aberration caused by the refractive and dispersive power 

 of the interposed medium deteriorates the performance of the 

 object-glass. 



