DESCRIPTION OF THK ACHROMATIC MICROSCOPE. 137 



SECT. 15. On viewing Transparent Objects by Day-light. 

 The best position of the instrument by day-light is 

 that represented at fig. 29. When the object-glass is of 

 long focus, a screen must be interposed to stop the light 

 from falling upon the object. If this precaution be not 

 taken, we could not obtain a correct knowledge of it, on 

 account of the confusion occasioned by the transmitted 

 and reflected light. They must not, therefore, be used 

 together. A small piece of black card placed upon its 

 edge, on the stage, will prevent the light from falling 

 upon a transparent body. 



SECT. 16. On viewing Transparent Bodies by Intermitting 

 Light. It has been ingeniously suggested, in order to 

 determine the number and arrangement of the rotatory 

 organs of Infusoria (such as the wheel-animalcule), to 

 illuminate them by a succession of galvanic or electric 

 sparks. By this means, an effect is produced similar to 

 that occasioned by the revolution of a cogged wheel when 

 viewed in a mirror. The reflection through the cogs 

 makes the wheel, although in rapid motion, appear 

 stationary. 



As there are several methods by which this effect can 

 be produced, it is unnecessary to enter upon the details 

 here, some persons preferring one way and some another. 

 In my own experiments, I have used the sparks emitted 

 by the electric magnetism of a wheel dipping in mer- 

 cury. 



