OF THE MICROSCOPE. 43 



of frame are sold, one containing two, and the other three 

 lenses, the latter being altogether the cheapest, in proportion 

 to the power furnished. The magnifying powers are about as 

 follows: With the lens of longest focus, five diameters; with 

 the lens of medium focus, eight diameters; with the lens of 

 shortest focus, ten diameters. When the lenses of shortest and 

 medium foci are combined the magnifying power is about 

 eighteen diameters; all three lenses together give a power of 

 twenty-five to thirty diameters. 



In using a combination of two or more lenses, the lens of 

 shortest focus should always be placed nearest to the object. 



As a dissecting microscope for botanical, entomological, and 

 physiological work, this instrument is very efficient and con- 

 venient. The glass plate is fitted into the stage so as to form 

 a cell capable of holding water, so that dissections may be 

 carried on under that liquid, or aquatic animals may be kept 

 alive and examined at leisure. The stage may also be turned, 

 so that the flat side will be up when so desired, in which posi- 

 tion it is most convenient for some purposes, such as dissec- 

 tions and the teasing out of tissues by means of needles. The 

 only serious defect in the Excelsior microscope is that it is not 

 sufficiently steady for ordinary work, the case which forms the 

 base or foot being, for portability's sake, made quite small. 

 This difficulty is, however, easily remedied by screwing the 

 case to a piece of pine board six inches long, four inches wide, 

 and three-quarters of an inch thick. A single small screw, 

 which does not deface the instrument, is sufficient, and when 

 the microscope is to be carried in the pocket it is easily de- 

 tached from its temporary stand. Its low price, $2.75, is a 

 strong recommendation. 



Raspail's Microscope. This was the instrument so 

 largely used by Raspail in his investigations into the structure 

 of plants, and having been fully described by him, it has been 

 called by his name, though Chevalier tells us that it was reallv 

 invented by Cuff, of London, r.nd ought to be called the Cuff 

 microscope. It consists of a pillar which screws into the top 

 of the box in which I he whole instrument packs when the micro- 

 scope is not in use. This pillar carries the mirror, and also a 



