achromatic compound Microscope. 173 



tain directions of the light they are liahle to show lines on an 

 object which do not really exist. 



It is observable, that, provided the aperture of the glass 

 remains open, the central pencil of light admitted behind many 

 transparent objects may be limited to a very small one with- 

 out greatly impairing their sharpness ; parallel lines or spots 

 spread closely over a flat surface, often remaining plainly visi- 

 ble in this case, which at a far less amount of contraction by a 

 stop behind the object-glass cannot by any management be 

 made to appear. The reason of this seems to be, that both 

 reflection and refraction of a part of the rays take place at such 

 objects, by which the pencil is spread out on leaving them to 

 a much increased angle in its progress to the glass. 



The relation between the aperture of microscopic object- 

 glasses, even of the same focal length, and the pencil of light 

 admitted by them, will vary much, according to differences in 

 their thickness, their combination, &c. ; and as aperture is va- 

 luable only in proportion to the pencil it admits, the latter 

 would seem to be the circumstance the more deserving atten- 

 tion of the two. It is so often erroneously estimated, that I 

 will mention a simple mode of ascertaining it, which will be 

 found pretty accurate. 



Fix a piece of paper on a table, and on it place the micro- 

 scope with its body horizontal, and one of the eye-pieces on; 

 set a candle on a level with it a few yards distant ; then 

 having directed the body of the instrument so far on one side 

 of the candle, as that the light from it shall bisect the field 

 vertically, leaving half of it dark, trace on the paper a line 

 corresponding to the side of one of the legs. Now, taking the 

 focus of the object-glass as a pivot, turn the microscope hori- 

 zontally to the other side of the candle, till the opposite half 

 of the field only is illuminated, and mark again on the paper 

 the position of the side of the leg. The measure of the angle 

 traversed, shown by the two lines, is that of the pencil of light. 



In the remarks which follow, the term correction is used to 

 imply the effect produced by the denser concave lens of a com- 

 pound object-glass upon the aberration of its convex. Thus, 

 as in a simple convex lens, the rays which pass through it near 

 ibe circumference, have their foci shorter than the more cen- 



