INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 703 



It will bo seen that the range in magnification is greater with some 

 lenses than with others, the dilfcrence increasing with the increase of 

 power, but with all it is so great, that scales made without taking it 

 into account would be worse than useless. Accuracy can only be 

 obtained by using the micrometer with the collar-adjustment at the 

 same point at which the object sketched has been examined, unless, 

 indeed, one were willing to take the trouble of compiling a table for 

 each objective, with the magnification noted for each division of the 

 collar through the whole range from " uncovered " to " covered." 



Zeiss's Microspectroscope. — This instrument, shown in Fig. 55 

 (half natural size), has, in addition to a comparison-prism and arrange- 

 ment for reducing the length of the spectrum, a micrometer by which 

 the position of bright or dark lines in the spectrum is determined by 

 a direct reading of their -wave-lengths. For this purpose a micrometer 

 scale is projected on the spectrum by reflection, by the divisions on 

 which the wave-lengths at every part of the spectrum (according to 

 Angstrom) can be directly read off in parts of a micromillimetre. 

 The divisions on the scale read to the second decimal place ; and the 

 third decimal may be easily estimated by the eye. For convenience 

 of recording observations there are lithographed sheets with ten 

 scales of wave-lengths enlarged to lOQ mm. 



Fig. 55. 



A is a shallow drum between the field- and cyc-glasses of an 

 achromatic eye-piece, and contains the 'SGravesande slits, comparieon- 

 prisni, &c. 



B is a cylindrical tube over the oye-piecc, and contains the Amici 

 prism. It carries the lateral tube C, which has a small adiromatio 

 objective at o at the focus of which at i is the micrometer scale. B 

 turns on the pivot m and is hold in the axis of the cyc-piccc by a 

 catch e ; by pressing this catch, B with all the parts attached to it 

 may be turned about the pin m, so that the eye-piece is free. 



