64 
No. 
screw Si the various colours of the spectrum may be made to 
successively traverse the field of vision. The screw serves to 
regulate the width of the slit 
MarJcs 
80.- 
Fig. 23. 
Centering Collar for the instruments 
Nos. 21, 22 and 23. 
Fig. 24. 
Micro-spectral Objective. 
'Micro-spectral Objective after Engelmann, for observ- 
ing and measuring the effect of the colours of the sjiectrum on 
microscopical objects (Botanische Zeitung, 1882, No. 26; Pflijger's 
Archiv, Bd. XXVII, p. 464, Bd. XXIX, p. 415). Slit mechanism, 
collimator lens, Amici prism and projection objective are com- 
bined in a tube about 77 mm in length, which fits below the stage 
concentrically with the axis of the microscope so as to project a 
real spectrum upon the preparation under observation. The edges 
of the slit are moved symmetrically by a screw with right and 
left hand threads, so that the middle of the slit remains stationary ; 
the divided head of the screw indicates the width of the slit as 
adjusted in 100"'* of a millimeter ; the length of the slit may be 
regulated on both sides by two slides acted upon by screws. — 
Ordinary objectives are used for projecting the spectrum, viz, 
according to the desired size of the spectrum, either A, B, C 
or D, which by the narrow gauge thread of the lens mounts may 
be screwed to the instrument just above the Amici prism. (Fig. 24.) 
134. 
