Stanley Evans: using a Dobson Spectrophotometer

Stanley Evans describes the Dobson Spectrophotometer which he used to measure ozone levels over Antarctica as part of the Royal Society's Antarctic Expedition, 1955-1957.

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The brief for the expedition was that they were to get south of seventy five degrees in the Weddell Sea longitudes. During the period of preparation, what I had to do was get acquainted with the Dobson Spectrophotometer. And this is also another glimpse of a – of a different world. Dobson, who was one of the professors, maybe the only one – no, Lindemann was also a professor at the Clarendon in Oxford. But anyway, he was by now interested in ozone, as I said, for purely academic reasons, you know. It’s just something there that we don’t know much about. At this stage, early ‘50s, he’d got to the instrument, which has lasted since that time in the same form. I daresay the technique of recording – you look at the – you look at the spectrum of the sun, either directly through heavy filters, or indirectly, the sky at the zenith, just the scattered light, or even at moonlight, or even in cloud, though you have to have different calibrations for the different conditions. But basically you’re looking at two or more spectral lines in the spectrum of the sun’s light. You had a choice of which lines you looked at. One of the two is one which is not significantly absorbed by ozone and the other one – the other line is one which is hopefully heavily absorbed by ozone, but these all have to be calibrated in the lab. So if you look at their relative strengths, you know how much ozone you’ve looked through. But what is amusing to think of now is – the way you measure their relative strength is to have an accurately calibrated – I think it’s called an optical wedge, meaning a piece of glass or whatever it is, the material, which is darkened like sunglasses from no darkening, gradually increasing across the width or length of it to something which is very dark. So you have one of these optical wedges inside the spectrophotometer and you have an adjusting knob when you’re looking through the eyepiece at these two lines and you move this optical wedge until the two are the same intensity, and then the wedge reading tells you the relative strength of these lines. Well, since this is a slightly subjective adjustment, and since it’s something that’s varying slightly with time, you want some means of averaging the readings over several minutes. You sit there, continually adjusting it so as to get a good mean value. And so you want a recording of the setting of this – [laughs] sorry I’m elaborating this point but I think it’s very amusing to go back to now. You want a means of recording the setting of this over a period of time so you can measure it, and can you believe, the recording medium was a disk of medal which you smoke over a candle so that it’s covered with soot. And the disc rotates slowly and you have a stylus, steel stylus, resting on it, which therefore draws a wiggly line as it rotates [laughs]. And this enables you to average – you look at the whole wiggly line and find its average value [laughs]. Marvellous, from the smoke of a candle [laughs].

  • Interviewee Stanley Evans
  • Duration 00:03:58
  • Copyright British Library Board
  • Interviewer Paul Merchant
  • Date of interview 5/31/2011
  • Shelfmark C1379/51

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