WATERPROOF PAPER Waterproof paper is used for mounting and wrapping up chemicals and sensitive material. A serviceable waterproof paper may be made by brushing a strong solution of Castile soap over some paper, and when nearly dry brushing over with a strong solution of chrome alum. This method of waterproofing answers well for fabrics generally.
(Fr., Longueurs des ondes; Ger., Wellenlangen) Light is considered to be an undulatory or wave-like motion in the ether, and the conven tional figures used to illustrate this are the fol lowing :— Assume that the light is travelling from left to right in each of the above figures, A, B, and C; the ether particles vibrate to and fro, and each excites its neighbour till the motion reaches the maximum of the crest of the wave at A, and then it vibrates in the contrary direction till it reaches the maximum of the trough at B, each ether particle merely moving to and fro between the planes bounding A and B. Now, a wave length is the distance from any two points in the same phase of movement, such as A and C, c and E, B and 1), and D and P. These points have been taken on the axis of the direction of the propagation of light, but any two other points could be measured from, such as A' and A" or a' and a", or 13 and 0", or b' and b". If the distance A C is a wave length, A B or B C is exactly half a wave length, and so on ; further, the length of the wave A to C in A red light is double that of C, the violet A to c, whilst in B green light, the length is midway between them.
The wave length of any pure spectrum is always constant, no matter what the source of light ; and it could be used as a definite standard of colour ; and it is by no means uncommon to see the expression that a colour is that of D I D ; that is to say, a colour can be matched by the spectrum ray, which lies exactly midway be tween the solar Praunhofer lines D and E.
There are several units of measurement or methods of writing the wave-length, which is usually abbreviated to A. The unit most gener ally adopted is the tenth-metre = i x iol° metre or the ten-millionth of a millimetre, which was the unit adopted by the great Swedish phy sicist Angstrom, one of the first to give a reliable map of the solar spectrum, and this is then abbreviated to A.U. (Angstrom's Unit) or t.m. It may also be expressed in millionths of a milli metre, called a milli-microne, and written 14 or in thousandths of a millimetre, called a micron, and written ; or it is also written as a ten thousandth of a centimetre or rob cm. So that we might describe the line as A 5.89616 x cm. = •00oo589616 cm. or A o•589616 /./ or A. 589'616 or A 5896.16 A.U. or t.m.
For all methods for measuring wave-lengths, except rough visual work, that of photographic coincidence is usually adopted.