Mr. Pritchard's Account of Test Objects for Microscopes. 337 



scope, assure us that it possesses certain good qualities. The 

 first division, as artificial stars, enamel dial-plate, wire gauze, 

 &c.*, which inform us of the state of their aberration, achro- 

 matism, centering, adjustment, curves, &c, I shall pass over, — 

 as many persons are not disposed to enter into a scientific 

 scrutiny concerning the causes of their demerits, and because 

 they are more applicable to engiscopes, or compound micro- 

 scopes, than to single and compound magnifiers, — and shall 

 content myself by giving some simple means of determining 

 effectiveness by means of the second division. 



(1.) Lepisma saccharina. — The insects of the families Le- 

 pismena? and Podurellse are comprehended in the order Thy- 

 sanoura of Cuvier and Latreille; they are small, frequenting 

 damp places, and are of various colours; they leap like fleas. 



The scales of these apterous insects must be taken from fresh 

 specimens, for, when long dead, they adhere so firmly to the 

 insect, that they cannot be detached without injury. 



Their longitudinal lines slightly radiate from the point of 

 insertion; they are readily seen, and appear flat or square, 

 like the indentations on some bivalve shells: these are the 

 prettiest scales I am acquainted with. There are other lines 

 in various directions, as shown in the drawing of a magnified 

 scale at fig. 1. Plate III. When the candle is so placed as 

 to bring out the latter strongest, and the scale is turned round 

 in the axis of the microscope in certain positions, they will 

 cease to appear connected. In this object it is the sharpness 

 and cleanness of the spaces that chiefly evince the goodness of 

 a microscope, for the longitudinal lines are easily developed. 



(2.) The Morpho Menelaus. — This butterfly is indigenous 

 to America, the wings are indented, and their superior surface 

 of a highly-polished blue colour. 



The imbricated scales from the centre of the superior side 

 of the wing are of a pale blue, mixt with others almost black. 

 The former are mostly broader than the latter, and are the test- 

 objects required ; they measure about one one-hundred and 

 twentieth of an inch in length. When viewed in a microscope, 

 they exhibit a series of longitudinal stripes or lines, as shown 

 in the magnified drawing, fig. 2. Plate III. Between these 

 lines are disposed cross striae, which, with the lines, give it 

 the appearance of brick- work. 



The microscope or engiscope under examination should 

 be able to make out these markings, with the spaces between 

 them, clean and distinct. The cross striae, which give the 



* For a particular account of these objects, see Dr. Goring's Memoirs 

 " On the Exact Method of, &c." p. 191. Mic. Cab. 



Third Scries. Vol. 2. No. 11. May 1833. 2 X 



