Until recently I had assumed that Rembrandt had used mirrors as part of the subject matter of his painting but apart from the evidence of the self portraits I could find nothing really specific.
Two paintings strongly suggested the use of a mirror. The first is Two Scholars Disputing dated 1628 where the two scholars' heads, but only the heads, confront one another very like many examples in the drawings. The other is St. Peter Denying Christ from 1660 where the two main characters have heads, hands and lighting in almost precise mirror imagery but for their features, one being the young maid-servant the other a rather elderly St. Peter.
Following a clue from G. Schwartz in Rembrandt: His Life his paintings that the National Gallery painting of The Adoration of the Shepherds dated 1646 had some features in common with the painting of the same subject and date in Munich but reversed, I set to work with maquettes. I found that there were indeed very many features of the subject of the Munich painting reflected in the National Gallery version. Some were reflected perfectly, others with sufficient in common with the original subject to be persuasive in the context.
This example is interesting from two points of view, it is my first clear example of reflections used in a painting and secondly it is by far the most elaborate group that I have found so far used in this way. For example even the cow forms a part of the reflection. True she needs to take three paces forward from her position in Munich to find her place in London but for a cow one might say that she has posed remarkably patiently. A ladder, a post and what I take to be a winnowing basket suggest that the whole scene might truly have been set up in a barn.
Of the human element I think we can say that the man with a lantern (in his right hand in Munich and in his left in London) is perfectly reflected but for the very minor repositioning of the other hand. All three of the Holy Family are very well reflected as a group though no single feature is identical because in the Munich painting they seem to be sitting just above ground level and in London they are distinctly higher. Two of the three shepherds that surround them in Munich are present in London but not in identical positions (this change is necessary if we are to see the Holy Babe).
The little boy on the extreme right in London occupies the position of the standing mother and child on the left in Munich but the little boy immediately in front of her carries the same hat as he is wearing in the London painting. The man to the right of the lantern bearer in Munich approximates to the two figures behind the Lantern bearer in London and the barely visible man behind them is again barely visible behind the cow in Munich.
When we compare reality and reflection we must not forget that the maquette represents live, human, models, who must have been present posed and reposed over a period of a week at least and therefore minor changes are to be expected. Added to which we may presume that Rembrandt's intention was to make two separate and distinct paintings and not to convince us that one was based on the reflection of the subject matter of the other.
Although the space and figures are by far the largest I have discovered so far the mirror is remarkably small (nine feet wide) which corresponds with that which is necessary for the drawing B Christ among the Elders and for Velásquez' Las Meninas. We must of course assume that it was made up of separate panes of glass mounted together or more probably of polished metal. Plate glass did not exist at this time.
A reflection in polished copper has a remarkably Rembrandtesque quality about it. The copper giving a lot of warmth to the light.
The Munich painting is accepted as a Rembrandt by the RRP, and the National Gallery painting is not, yet both were observed from the same viewpoint. Most sensible people would therefore accept the opinion of previous generations of connoisseurs and happily welcome back into the fold this exquisite little painting.