Suggesting 'The difference in aspect, as well as the occasional difficulty of finding this interesting spot is highly curious', (13) Birt secured visual observations of the region on 1862 January 3, when, 'Schroter's Alhazen, at first sight, appears to have somewhat the appearance of a crater, the West edge being high, but the East much lower'.
Convinced he had successfully recovered Schroter's Alhazen, Birt reinforced his findings by outlining further observations made on the following evening, 1862 January 4, when, under very fine definition, 'the real character of Schroter's Alhazen well seen; the Southern termination of the two mountain ranges was seen to be quite separate the one from the other, and the level surface passing between them.
7cm achromatic refractor) under very favourable conditions, he ascertained that 'Schroter's Alhazen is really a crater, situated on the surface between the two ranges of mountains, and but slightly depressed below it.
Interpretation of the observations carried out by Kunowsky and Kohler is of fundamental importance in considering Schroter's Alhazen, and I was fortunate to be able to consult a translation made by Dr William Sheehan of the relevant text from Der Mond which deals with Alhazen.
In Bode's Jahrbuch for 1825, there appears, among many admirable remarks by Herr Kunowsky, a comment to the effect that Alhazen was no longer seen.
Most assuredly this valley is the depression Alhazen seen by Schroter.
Kohler surmised that the craterform appearance of Schroter's Alhazen was dependant upon changing conditions of illumination, Beer & Madler suggesting that this hypothesis could at least 'explain away the changes on which Schroter, Gruithuisen placed such emphasis, the region remains, nevertheless, one deserving careful study .
He mentions Schroter's Alhazen on two occasions; on 1845 February 11 he records that 'Schroter's Alhazen in darkness level resembles Edge' and later on June 20 of the same year he described 'Schroter's Alhazen ring mountains with a grey interior'.
In the first instance I was keen to establish if Schroter's positioning of these features could be relied upon as an accurate representation comparable with modern charts, and perhaps establish if there was an identifiable feature in the position of his Alhazen.
An examination of the photograph indicates there are two obvious detached mountain masses on the eastern (IAU) 'shoreline' of the Mare, the largest of which appears to be located where Schroter depicts his crater Alhazen.
It is apparent that successful observation of a crater-like object as Schroter drew it is dependent upon a certain amount of interpretation, however there is a secondary issue which emerges whilst considering later investigations, an issue which is of no lesser interest than the actual charting of Alhazen itself.