"How so?"
"Well, you've probably heard of the telephoto lens—a sort of long distance microscope, to use very colloquial language. I have seen telephoto pictures of the Matterhorn, taken five or six miles away, in which you could make out the actual geologic texture of the rocks.
"But, of course, there must be plenty of light on the object to get clear definition. On the same principle, one can stand inside a room and see everything outdoors with perfect distinctness. It's a very different thing, trying to look into a room from without. The visibility is low, as they say, and you don't get much."
"Yes, I understand that."
"Again there are optical lenses specially designed to make the most of poor illumination. A familiar example is the sailor's night-glass.
"You guess what I'm coming to. This particular lens has the telephoto range, and, at the same time, it works with the minimum of illumination. Never saw anything like it before, and it would be worth a fortune in the binocular field."
"Show me."
Chalmers Warriner rested the long glass on a fence post, ranged it on the open door of the barn nearly three thousand yards away, and did some preliminary focussing and other adjustments. He took a look, and then invited me to do the same.
It was truly marvellous! It seemed as though I were standing on the very threshold of the barn and looking inside. I recognized Adam Lake, the field foreman, working on the engine of a small tractor. In the background, Zack was oiling a set of harness. The details were astoundingly distinct.
"It's evident now," continued Warriner, "that the iron trough at Thaneford's observation point was intended to support a telescope such as this. The instrument is too long to hold steadily in the hand, and it had to be ranged precisely on the two-foot opening of the pridella. It was therefore possible to sit comfortably concealed on Sugar Loaf, and keep accurate tab on whatever was passing in Francis Graeme's library; provided, of course, that one of the pridellas was open. Even this wonderful lens could not penetrate stained glass. It isn't an X-ray apparatus."