278 optics. 



true focal distance, and the light be seen reflected 

 from the speculum strongly upon the object j by 

 which means it will be shown in a manner sur- 

 prisingly distinct and clear. And, for this pur- 

 pose, the light of the sky, or of a candle, will 

 answer. 



Though this microscope is principally intended 

 for opaque objects, which are very difficult to exa- 

 mine, yet transparent ones may also be viewed by 

 it : observing only, that when such come under 

 examination, it will not always be proper to throw 

 on them the light reflected from the speculum : 

 for the light transmitted through them meeting 

 the reflected light, may, together, produce too 

 great a glare. A little practice will teach how to 

 regulate both these lights to good advantage. 



In Wilson's single pocket microscope, the body is 

 made either of brass, ivory, or silver. 



C (Fig. 5.), is a long fine-threaded male screw, 

 that turns into the body of the microscope B. 



D, a convex glass at the end of the said screw. 



There are two concave round pieces of thin 

 brass, with holes of different diameters in the mid- 

 dle of them, to cover the said glass, and thereby 

 diminish the aperture when the greatest magnifiers 

 are employed. 



E E, three thin plates of brass within the body 

 of the microscope, one whereof is bent semicir- 

 cularly in the middle, so as to form an arched 

 cavity for the reception of a tube of glass, whereas 

 the two flat plates are to receive and hold the 

 sliders between them. 



At G, the other end of the body of the micro- 

 scope, a hollow female screw is adapted to receive 

 the different magnifiers. 



