482 MANIPULATION-. 



from an argand lamp at night; direct rays should be first 

 employed, and the object brought weU into focus; if it be a 

 lined one, the concave mirror should be turned in various di- 

 rections, in order that the lines may be distinctly seen; but 

 the light should not be too oblique, as then fallacious appear- 

 ances may be produced; if the achromatic condenser be re- 

 quired, the plane mirror should be used; and when the object 

 is in focus, the illuminating lens should be moved up or down 

 gently, to see at what point the definition is the best. If the 

 power to be tested be an eighth or a twelfth, and the object a 

 very minute one, a half-inch should be first used to find it out 

 and bring it into the centre of the field; the high power may 

 then be substituted for the lower one, and if the axes of the 

 two glasses coincide, the object wiU be found in the centre of 

 the field, or very near it. It is always a tedious matter to find 

 a minute object in a slide with a high power, unless a small 

 circle be marked around It; but in practice it wiU be found 

 most convenient first to examine the sHde with a half-inch or 

 inch, and to bring into the centre of the field of view the ob- 

 ject required. Mr. GiUett adopts a very excellent method; he 

 searches over all the objects contained in a slide, and paints a 

 circle around the best specimens, and makes an enlarged draw- 

 ing or chart of the slide on paper with aU the circles, and within 

 each circle a magnified representation of the objects contained 

 in it ; if the slide and the chart be compared, the circle within 

 which the best specimens are contained can be placed in the 

 field of view without much difficulty. 



In testing the merits of any two glasses of equal power, the 

 same illumination and object should be employed with each, 

 and the only way of getting a measure of their relative value 

 is to select a test that can be resolved by both; and that glass 

 which shows the lines darkest, and all elevations the most 

 prominent, and the spaces between them the clearest, may be 

 considered to perform the best. Particular care should be 

 taken in the management of the illumination, so that the rays 

 be not too obhque, as it often happens that projections are 

 shown as depressions, and depressions as projections. Objects 

 the intimate structure of which it is difficult to define, should 



