Bagwax, who since that little check had been standing perfectly still, with his open bag in his hands, at once began his work.  The plain before him was immense, and he was able to marshal all his forces.  In the centre, and nearest to Sir John, as he sat in his usual chair, were exposed all the Mays ’73.  For it was thus that he denominated the envelopes with which he was so familiar.  There were 71’s, and 72’s, and 74’s, and 75’s.  But the 73’s were all arranged in months, and then in days.  He began by explaining that he had obtained all these envelopes ‘promiscuously,’ as he said.  There had been no selection, none had been rejected.  Then courteously handing his official magnifying-glass to the barrister, he invited him to inspect them all generally,—­to make, as it were, a first cursory inspection,—­so that he might see that there was not one perfect impression perfect as that impression on the Caldigate envelope was perfect.  ‘Not one,’ said Bagwax, beating his bosom in triumph.

‘That seems perfect,’ said Sir John, pointing with the glass to a selected specimen.

’Your eyes are very good, Sir John,—­very good indeed.  You have found the cleanest and truest of the whole lot.  But if you’ll examine the tail of the Y, you’ll see it’s been rubbed a little.  And then if you’ll follow with your eye the circular line which makes up the round of the postmark, you’ll find a dent on the outside bar.  I go more on the dents in those bars, Sir John, than I do on the figures.  All the bars are dented more or less,—­particularly the Mays ’73.  They don’t remain quite true, Sir John,—­not after a day’s fair use.  They’ve taken a new stamp out of the store to do the Caldigate envelope.  They couldn’t get at the stamps in use.  That’s how it has been.’

Sir John listened in silence as he continued to examine one envelope after another through the glass.  ’Now, Sir John, if we come to the Mays ’73, we shall find that just about that time there has been no new stamp brought into use.  There isn’t one, either, that is exactly the Caldigate breadth.  I’ve brought a rule by which you can get to the fiftieth of an inch.’  Here Bagwax brought out a little ivory instrument marked all over with figures.  ’Of course they’re intended to be of the same pattern.  But gradually, very gradually, the circle has always become smaller.  Isn’t that conclusive?  The Caldigate impression is a little, very little—­ever so little—­but a little smaller than any of the Mays ’73.  Isn’t that conclusive?’

’If I understand it, Mr. Bagwax, you don’t pretend to say that you have got impressions of all the stamps which may have been in use in the Sydney office at that time?  But in Sydney, if I understand the matter rightly, they keep daily impressions of all the stamps in a book.’

‘Just so—­just so, Sir John,’ said Bagwax, feeling that every word spoken to the lawyer renewed his own hopes of going out to Sydney,—­but feeling also that Sir John would be wrong, very wrong, if he subjected his client to so unnecessarily prolonged a detention in the Cambridge county prison.  ’They do keep a book which would be quite conclusive.  I could have the pages photographed.’