Ch. XI] MODELS FROM SERIAL SECTIONS 419 



A concave mirror or reflector behind the lamp is of considerable 

 advantage, for the Ught which extends backward is reflected forward 

 to the condenser and is thus available for illuminating the object. 



§ 678. Large condenser for drawing. — If the three-lens condenser 

 is used (fig. in), and it is much to be preferred, the second element 

 which converges the parallel beam should be of long focus. One of 

 38 cm. (15 in.) focus has been found very satisfactory. The reason 

 for using the long focus lens is discussed in Ch. VI, § 297, fig. 115. 



If a two-lens condenser is used the second element should also be 

 of longer focus than for ordinary magic lantern work, for the same 

 reason as for the three-lens condenser. 



§ 679. Drawing with the small projection outfit. — If one has no 

 large projection outfit, drawings for models and for pubhcation can 

 be made very satisfactorily with the 6-volt lamp as follows: It is 

 a great advantage to have the lamp in one of the metal lanterns 

 like those used for daylight glass (fig. 37-38), then scattered light wiU 

 be avoided. There should be a condenser like that used for the small - 

 arc lamp (fig. 49). It should be over one of the daylight glass open- 

 ings and of course centered with the lamp filament. If it were in a 

 tube which permitted of a hmited amount of movement, as with the 

 condenser of the small arc lamp, it would be of much advantage. 

 As the microscope must be horizontal and is ordinarily raised to 

 make the drawing distance 250 mm., the lantern containing the 6-volt 

 lamp must be supported on a box or block to bring the filament of 

 the lamp in the optic axis of the microscope. 



When horizontal the microscope is unstable; hence a weight or 

 better a clamp is put over the feet to hold the microscope firmly 

 so that when once centered it will not move easily. A table with 

 the drawing shelf on the legs is very convenient for getting the de- 

 sired magnification (fig. 247). 



§ 680. Relative position of the lamp and microscope. — This can 

 be as with the small drawing outfit and arc lamp (fig. 113), or it can 

 be put in line, as with the large outfit. If in line (fig. in) the mirror 

 is not used, and care must be taken to get all parts hned up to one 

 axis. With the mirror slight deviations from centering can be over- 

 come by inclining the mirror accordingly. 



