$$\require{cancel}$$
In this chapter I use the standard terms, but I point out when necessary where astronomical usage sometimes differs. In particular I shall discuss the astronomical usage of the words "intensity" and "flux" (which differs from standard usage) in sections 1.12 and 1.14 . Standard usage also calls for SI units, although the older CGS units are still to be found in astronomical writings. Except when dealing with electrical units, this usually gives rise to little difficulty to anyone who is aware that $$1 \ \text{watt} = 10^7 \text{erg s}^{-1}$$. Where electrical units are concerned, the situation is much less simple.