122 JAMES CLERK MAXWELL. [CHAP. V. 



Thus the parabola rolled on a straight line traces a Catenary 

 with its focus, an easy way to describe the Catenary. Pro- 

 fessor Wallace just missed it in a paper in the Eoyal 

 Society. 



After props come optics, and principally polarised light. 



Do you remember our visit to Mr. Nicol ? I have 

 got plenty of unannealed glass of different shapes, for I 

 find window glass will do very well made up in bundles. I 

 cut out triangles, squares, etc., with a diamond, about 8 or 9 

 of a kind, and take them to the kitchen, and put them on a 

 piece of iron in the fire one by one. When the bit is red 

 hot, I drop it into a plate of iron sparks to cool, and so on 

 till all are done. I have got all figures up to nonagons, 

 triangles of all kinds, and irregular chips. I have made a 

 pattern for a tesselated window of unannealed glass in the 

 proper colours, also a delineation of triangles at every 

 principal inclination. We were at Castle-Douglas yester- 

 day, and got crystals of salt Peter, which I have been 

 cutting up into plates to-day, in hopes to see rings. There 

 are very few crystals which are not hollow-hearted or filled 

 up with irregular crystals. I have got a few cross cuts 

 like { ) free of irregularities and long [wedge-shaped] cuts 

 for polarising plates. One has to be very cautious in sawing 

 and polishing them, for they are very brittle. 



I have got a lucifer match box fitted up for polaris- 

 ing, thus. The rays suffer two reflections at the polar- 



ising angle from glasses A and B. Without the lid 

 it does for an analysing plate. In the lid there is set a 

 plate of mica, and so one observes the blue sky, and turns 

 the box round till a particular colour appears, and then a 

 line on the lid of the box points to the sun wherever he 

 is. Thus one can find out the time of day without the sun. 



