46 



COLOUR PHOTOGRAPHY OF ROCK SECTIONS UNDER 

 POLARISED LIGHT. 



J. W. PATTERSON, F.C.S., 



West Hartlepool. ..... 



The photography of rock sHces under the microscope when 

 polarised hght is used has always been disappointing, the 

 beautiful polarisation colours being incapable of registration 

 by the process. The ' Plaques Autochromes ' of Messrs. 

 A. Lumi^re & ses Fils have now made it possible to reproduce 

 these colours with certainty and truthfulness. In obtaining 

 them the usual arrangement of microscope and camera was 

 used. An arc light was the source of illumination, and various 

 objectives from to 4'' were employed. In some cases no eye- 

 piece was used ; in others a low power ' A ' eyepiece was found 

 useful. In the microscope, a Swift's Petrological, the analyser 

 slipped in and out of the tube, and the polariser was in the usual 

 position beneath the stage. When a Bertrand Lens was required, 

 this was in the tube below the analyser, and an eyepiece was 

 needed. The hemispherical lens required for the convergent 

 light fitted into the stage. The yellow screen, necessary for use 

 with these plates, was so fixed in the camera that the light 

 passed through it last, unless an eyepiece was being used when 

 the screen was placed in front of the polariser. 



The instructions issued by the makers were followed, and 

 there seems good reason for adhering to their rule to develop 

 tor exactly 2 J minutes in complete absence of light. The 

 manipulation in total darkness is so simple, that no one can feel 

 any inconvenience, and the extremely delicate and sensitive 

 film seems to require this care. Notwithstanding its thinness, 

 the film shows no tendency to ' frilling,' and takes its different 

 treatments in a most docile manner. The exposure required 

 for these plates is not great, and. poor results are obtained by 

 over-exposure, which is certainly the unpardonable sin here, 

 for detail is lost, and feeble colours result. Much light is cut 

 off by the rather numerous optical appliances, especially the 

 polariser ; but from 17 to 20 seconds was found to give the best 

 results with the apparatus used. Correct exposure seem.s to be 

 an important detail in the process, and as the time for develop- 

 ment is fixed by other considerations, the reason is plain. ^ 



Naturalist, 



