764 



SIMPLE MICKOSCOPE. 



the fatigue attendant on long-continued investigations, and the small 

 field of view. In the simple microscope, glasses of the following 

 focal lengths may be employed — viz., 1^ inch, f, J, i; and, if very 

 minute objects are to be examined, of 1-lOth, l-30th, or l-40th of an inch. 

 For examining minute plants, such as Diatomacese and Desmidise, 

 during an excursion, it is useful to have a simple microscope similar 

 to that represented in fig. 945. It consists of a WoUaston's 

 doublet, fixed in a round plano-concave brass disc (fig. 945, 1, a), 



'6 



1 Fig. 945. 2 



attached to a small brass handle (fig. 945, 1, i). For ordinary botanical 

 purposes a lens magnifying 65 to 70 diameters is enough ; but the 

 lenses may be procured with a power of 150 to 220 diameters. On 

 the plane side of this brass disc there is a ring of silver (fig. 945, 1, c), 

 in which a thin piece of glass is fixed, also supported by a brass 

 handle, which acts as a spring, so as to keep the two rings in contact. 



Fig. 945 represents Dr. Gairdner's portablef simple'microscope. In 1 tliere is given a 

 front view of tlie instrament, showing the posterior silver ring, c, enclosing a piece of thin 

 glass, separated and tnmed aside from the disc, a, containing the doublet, to which the 

 eye of the observer is applied. 2 exhibits a lateral view of the instrument, with the screw, 

 d,ihy means of which the handles are separated or approximated, so as to bring the object 

 into focus. 



