TRANSACTIONS OF SECTION C. 725 



moraine of tlie Irish Sea glacier. Erratics from the Lake district and Scotland, 

 and flints and shell fragments from the bed of the Irish Sea, distinguish this 

 moraine from others farther north. No striae and no shell fragments have been 

 discovered to the north-east of this line of drift hills, while to the south and west 

 of it both striated rock surfaces and shell-bearing drift are ahiuidant. Although 

 frequently levelled down by natural and artificial agencies, in many places these 

 drift hills retain the typical features of a moraine. 



8. On a sim'ple metJiod of projecting upon tlie i>creen Microscopic Bock 

 Sections, both by ordinary and by polarised light. ]3y E. P. Quinn. 



Knowing the difficulty experienced in pointing out to students any particular 

 crystal in a rock section when viewed with the microscope direct, I attempted to 

 project the images on the screen, and by the aid of comparatively simple apparatus 

 met with very gratifying success, both with ordinary and with polarised light. 



The tube of the microscope was screwed out and replaced with a cork, through 

 which a bole had been cut to carry the ordinary one-inch micro-objective, and 

 behind it the analyser of the microscope. The polariscope and rock section 

 occupied their usual position as when used with the microscope in the ordinary 

 way. The microscope stand being inclined into the horizontal position was placed 

 in front of the object lens of the lime-light lantern. The object lens of a lantern 

 usually consists of a combination of two lenses. If so the back lens is taken out 

 and the front lens only used, acting as an extra condenser, concentrating the light 

 upon the rock section and causing it to pass through the polariser and the 

 analyser. 



A little adjustment of the light was required to get it well through both 

 polariser and analyser, but this with a little care was soon done, and a bright 

 picture, several feet in diameter, was projected upon the screen, showing the crystals 

 well defined and exhibiting very strikingly the changes of colour, &c., characteristic 

 of the crystals when viewed by polarised light, and in such a manner as to be well 

 seen by a number of people at once, and also allowing the lecturer to readily point 

 out any particular crystal or crystals to which he desires to draw the attention of 

 his audience. As the optical axis of the lantern and microscope did not coincide, 

 the lantern was placed on a board provided with four levelling screws, with which 

 the necessary adjustments were readily made. 



JMuch better effects may be got if the ' Prazmowski ' form of prisms made by 

 Zeiss are used mstead of the usual Nicolls prisms, on account of their greater 

 aperture and shorter length, and the most brilliant results with the one-inch ob- 

 jective of fifty angular apertures by Wray of London. 



