NEGATIVE MAKING 75 



ordinary transmitted light, in addition to 

 greatly increasing their beauty. It is usually 

 effected by means of a paraboloid of glass 

 carried by the sub-stage, which reflects to its 

 focus the rays which fall upon its internal sur- 

 face. It is useful with objectives of moderate 

 angle and power only. I have not succeeded 

 very well with any above TO inch, but others 

 seem to have done better. The exposure in 

 photographing is greatly lengthened as com- 

 pared with that required for the same object 

 illuminated by direct transmitted light. No 

 change is required in the optical arrangement 

 we have used in medium power work, except- 

 ing substitution of a paraboloid for the sub- 

 stage condenser then employed. By this 

 means the negative of Fig. 13 was produced. 

 It is a group of the anchors and plates of 

 Synapta (a small marine animal), in the center 

 of which may be seen a wheel of Chirodota 

 violacea, belonging to the same order of holo- 

 thuridae. These are calcareous, without color, 

 and not conspicuously brilliant objects when 

 viewed by ordinary transmitted light; but, 

 seen brightly illuminated upon a black ground, 

 they are exceedingly beautiful, as the print 

 well shows. In making this negative the ob- 

 jective was a Beck first-class |, without eye- 



