Why would you ever want to place a microphone behind the instrument?
There are two basic rules of microphone placement. The first is to point the microphone at the sound source. The second is that the microphone should be closer than the natural listening distance. There are of course a few exceptions to these rules, but the exceptions are indeed few.
One non-exception is the French horn, known to classical musicians simply as the horn. This is a peculiar instrument that directs sound to the rear of the player.
Why no-one ever thought to design a forward-firing French horn is a mystery.
If you need to record a solo horn, or a French horn section as shown in the picture, then it makes sense to put the microphone behind the player so that it picks up the sound directly.
But when the horns are playing in the context of an orchestra, there is more to think about.
Firstly, the audience would naturally hear the reflected sound of the horn rather than the direct sound. So you might decide that no special treatment is necessary. Classical orchestra miking is done by section rather than by instrument. So the entire brass section could be covered perfectly well by two microphones, the French horns simply taking their chances.
This can work. The horns will be at a level that is subjectively on a par with the rest of the brass. However, where all sounds well in a concert, the microphone will definitely give away the fact that the horns are pointing in the wrong direction.
So you could decide to add an extra mic or two behind the horns. This will work too, with careful setting of levels, and perhaps a little EQ to 'dull down' the horns to a more natural perspective.
Another more surprising solution is to place a reflective surface behind the horns. This could be a sheet of wood, or in fact any hard flat surface that can be moved into a suitable location.
The French horn problem is one that is perfectly straightforward to solve. However, one does have to wonder when the forward-firing French horn will be invented.