WITH THE MICROSCOPE. 287 



j-y of an English inch, magnified 250 linear. 



ToVo f an English inch, magnified 700 linear. 



of an English inch, magnified 2800 linear. 



SCO. Of the Covering Glass. The cover may be made of a very- 

 thin plate of mica, but glass possesses several advantages. Messrs. 

 Chance, of Birmingham, have lately succeeded in manufacturing in 

 quantity glass sufficiently thin for the y^ri- This is supplied by Messrs. 

 Powell and Lealand. For mere examination of specimens, thin 

 plates of mica answer well, and they may even be used for mounting 

 the preparation ; but as it is difficult to clean the surface without 

 scratching it, it will be found better to use thin glass circles as the 

 covers of specimens which are to be kept permanently. 



An instrument for measuring the thickness of the thin glass kindly 

 lent to me by Mr. Brooke, is represented in fig. 343, pi. LVI. The 

 method of using it is too obvious to require explanation. The marks 

 round the stem indicate tenths of a millimetre. The thickness 

 of the ordinary thin glass is about 3 or 4 tenths of a millimetre, that 

 for the Y^-th objective under two tenths, and that suitable for the 

 y^th about til of a millimetre, or less than T i_ of an English inch. 



361. Illumination of Objects Magnified by very High Powers. 

 In using the sixteenth, twenty-fifth, and fiftieth, it is of the utmost 

 importance that the illumination should be good. As already stated, 

 the ordinary concave mirror gives light enough for the one twenty- 

 fifth objective; but a light of greater intensity and far superior in 

 quality may be obtained by other methods. After having tried a 

 great many different plans, I have decided in favour of the illumina- 

 tion obtained from a round wicked paraffin lamp, pi. XI, figs. 

 47, 48, brought to a focus by a condenser. The ordinary con- 

 denser answers very well if to the front glass is fitted a cap made of 

 very thin brass having a perfectly round central aperture less than 

 the -g'pth of an inch in diameter. Reiner's eye-piece, as before ob- 

 served, p. 24, gives the brightest illumination, and if covered with 

 a cap having an aperture of about T ^ of an inch, the character of the 

 light is all that can be desired. Sufficient light is afforded by this 

 arrangement when the tube of the microscope is lengthened so as to 

 give an amplifying power equal to 10,000 linear. The universal con- 

 denser referred to in p. 25, with a stop upon the surface of the front 



