Wolff — Apparatus for the Geological Laboratory. 357 



parallelism by eye, the precaution of turning the slide will 

 generally keep the saw from cutting through the specimen 

 into the glass. The saw must be kept wet by dropping water 

 or by a sponge. When it is stated that slices have been sawed 

 in the laboratory from ^ to T \-^ of an inch thick it will be 

 plain how easily this accident may happen without precautions, 

 but that when successful this method saves much time and 

 labor in attaining the ultimate thickness (about -^\^ inch). 



The Arc Light for Projection. 



In microscopic projection, especially with high powers and 

 polarized light, the lime-light is insufficient for a projection of 

 any size, while with the arc light, objectives of 4 mm focus and 

 less can be used. The following arrangements are made in 

 the laboratory for this purpose. 



A 1 Kilowatt dynamo, requiring about 1^ H. P., is run by 

 a belt from the motor in the basement and the current carried 

 to two lecture rooms above. At full load the dynamo gives 18 

 to 21 amperes current at 55 volts, while by varying the load 

 and regulating the current to the fields, from 80 to volts can 

 be obtained. For microscopic projection a large Zeiss micro- 

 photographic apparatus was available, but any petrographical 

 microscope with condenser and water chamber can be used 

 with a suitable arc lamp. Special projecting microscopes such 

 as those made by Newton & Co., London, are said to give ex- 

 cellent results. We use the Schuckert projection lamp which 

 belongs with the Zeiss apparatus, in which the carbons are in- 

 clined so that the crater of the positive carbon radiates the 

 light to the condenser. The lamp is adjusted for 45 volts 16 

 amperes but runs perfectly with 50 volts and 19 or 20 amperes 

 giving from 2500 to 3000 c. p. It is perfectly steady and 

 self -regulating, so that when once started it burns without 

 attention. It can be run with least trouble from a storage bat- 

 tery of 25 to 30 cells, thus avoiding inequalities of current 

 due to the running of the dynamo. The imported carbons 

 cost about 50 a pair and burn two or three hours. A variable 

 German silver wire resistance with sliding contact is intro- 

 duced into the circuit near the lamp, so as to give some con- 

 trol over the current in case of variation in speed of the 

 dynamo. A switch throws the same current into 20 incandes- 

 cent lights for lighting the room when the arc is not in use. 

 With this arrangement projections of rock slides in polarized 

 light are shown to the class in illustration of the lectures. The 

 screen, which consists of a plaster surface cast on plate-glass 

 and mounted in a frame, stands at 12 feet from the microscope 

 and the projections are then about 2 ft. 6 in. in diameter, cor- 



