Mr. Pritchard's Account of Test Objects for Microscopes. 34-1 



the diamond beetle (Curculio impcrialis) is shown at fig. 12. 

 Plate III.* The lines and markings on certain vegetable tis- 

 sues, and many others too numerous to name, may also be 

 employed as proof objects. The reason for making a selection 

 of those above described, has been in order to render the task 

 of judging of the merits of an instrument by different indivi- 

 duals more simple and satisfactory, so that by the assistance 

 of the drawings, and a sample of the objects, they may ascer- 

 tain the quality of an instrument without the trouble of com- 

 paring it with others, which are often difficult, and sometimes 

 impossible, to procure. 



Defnition. 



The defining power of microscopes and engiscopes depends 

 on their capability of collecting together all the rays from any 

 one point of the object, or, in other words, their freedom from 

 aberration, and is independent of their penetration; for, if we 

 take an engiscope and view a lined object with the aperture of 

 the objective, as it is usually sold in the shops, its defining 

 power may be very fair ; but if we enlarge the aperture so as 

 to enable us to develope the lines which it will then accom- 

 plish, the defining power of the instrument will be injured to 

 such an extent as to render the outline quite confused. The 

 great desideratum, then, in microscopes and engiscopes, is to 

 obtain these two qualities combined, which, however, is only 

 rarely attained. 



Cylindrical or spherical bodies appear the best suited for 

 ascertaining the goodness of an instrument, as regards defini- 

 tion ; and the following examples, which are prefaced by re- 

 marks on the method of illuminating them, I deem sufficient 

 for this end. 



In the preceding class of objects, oblique diverging rays ap- 



* The scales from the body of the diamond beetle, either as transparent 

 or opake objects, are by far the most brilliant, in point of colour, of any 

 of the lined class. In viewing them as opake objects, with single lenses, 

 in order to exhibit the lines, the scale must be brought a little within the 

 focus, and the illumination carefully arranged. As you cannot exhibit them 

 with single lenses of a one twentieth or one thirtieth of an inch focus with- 

 out using silver cups, it is difficult to procure oblique light. As transparent 

 objects, they are much easier managed. They present a mottled sort of 

 colour, composed of the brightest carmine, mixed with purple, blue, and 

 yellow, and their lines are distinctly seen, as shown in fig. 12. As the 

 lines on some of these scales are of easy resolution, it will not be advisable 

 to trust every specimen as a test. The small ones from the legs of the 

 Brazilian beetle are the most difficult, and many oi' these require the most 

 rigid adjustment of the focus and illumination to resolve the lines, and the 

 slightest tremor, though not enough to occasion any sensible dancing (as a 

 carriage at a distance), is sufficient to render them invisible. 



