Note 80 — PART SECOND. NOTES ON THE ILLUMINATION OF LIGHTHOUSES, WITH SHORT NOTICES OF THEIR EARLY HISTORY. (continued 3)

This quantity is deduced from observations made by my friend Mr James Gardner, while engaged on the Ordnance Survey, and may be regarded as the extreme limit of visibility, under the most favourable circumstances as to the state of the atmosphere and also the contrast of colours. The observed object, also, was a pole, not a round disc; and it is familiar to every one accustomed to view distant objects, that vertical length is an important constituent in their visibility.