THE OTTAWA NATURALIStt 



VOL. XXIX. DECEMBER, 1915 No. 9 



THE USE OF GUM DAMAR IN PALEOHISTOLOGY. 



(With Notes on the Genus Benthopecten.) 

 By George H. Hudson, Plattsburgh, N.Y. 



In the study of the detail of opaque objects with the simple 

 or compound microscope, there are some very decided advan- 

 tages to be obtained through covering the object with some 

 transparent meditim that may be used to hold a cover glass in 

 position. The writer has long used a solution of gum damar 

 in benzol for this purpose, and whether the mounting was for 

 temporary observation, for drawing under camera lucida or 

 for photomicrographic work, the results were often of surpris- 

 ing value. For instance, he was enabled by this method to 

 obtain a microphotograph which without retouching was used 

 for the production of a figure (1911, plate VI, fig. 1) showing 

 clearly the sutures surrounding the radianal of Palaeocrinus 

 striatus, Bill. Billings stated that he could not make out the 

 sutures in this region, and so left it blank in his published 

 analysis. Bather, in Lancaster's "A Treatise on Zoology," 

 Part III, p. 1 72, gives an analysis that for this region is in error. 

 How great a help this process is in revealing sutures may also 

 be seen by comparing (1911) figures 2 7 and 28 on page 2 52. 

 The writer will here give reasons for the character of the results 

 obtained, present other advantages of the method, and give 

 briefly a description of the process as he uses it. 



Suppose that we make the attempt to photograph a printed 

 page through a sheet of ground glass placed directly over it. 

 Much of the incident light will be reflected and scattered. Such 

 of these rays as enter the lens will tend to produce a uniform 

 fog over the whole negative. They are from the ground glass 

 surface .and not from the covered paper. That portion of the 

 light which reaches the printed surface cannot return without 

 being subjected to both reflection and refraction on accoimt 

 of the many minute angles presented by the ground surface 

 through which it must pass. This tends to give us numerous 

 overlapping images. If now we will wet, oil or varnish the 

 ground surface we shall cut down its reflecting power to a 

 marked degree. The more nearly alike the indices of refrac- 

 tion of the two transparent media the greater will be the amotmt 

 of light received by the lens from the covered object, and the 

 sharper will be the negative secured. 



