I followed, and a few moments later, across the campus in his laboratory, he was working quickly over an X-ray apparatus.  He had placed the letter in it.

“These are what are known as ‘low’ tubes,” he explained.  “They give out ‘soft rays.’” He continued to work for a few moments, then handed me the letter.

“Now, Walter,” he said, “if you will just hurry back to the museum and replace that letter, I think I will have something that will astonish you—­though whether it will have any bearing on the case, remains to be seen.”

“What is it?” I asked, a few minutes later, when I had rejoined him, after returning the letter.  He was poring intently over what looked like a negative.

“The possibility of reading the contents of documents inclosed in a sealed envelope,” he replied, still studying the shadowgraph closely, “has already been established by the well-known English scientist, Doctor Hall Edwards.  He has been experimenting with the method of using X-rays recently discovered by a German scientist, by which radiographs of very thin substances, such as a sheet of paper, a leaf, an insect’s body, may be obtained.  These thin substances through which the rays used formerly to pass without leaving an impression, can now be radiographed.”

I looked carefully as he traced out something on the negative.  On it was easily possible, following his guidance, to read the words inscribed on the sheet of paper inside.  So admirably defined were all the details that even the gum on the envelope and the edges of the sheet of paper inside the envelope could be distinguished.

“Any letter written with ink having a mineral basis can be radiographed,” added Craig.  “Even when the sheet is folded in the usual way, it is possible by taking a radiograph stereoscopically, to distinguish the writing, every detail standing out in relief.  Besides, it can be greatly magnified, which aids in deciphering it if it is indistinct or jumbled up.  Some of it looks like mirror writing.  Ah,” he added, “here’s something interesting!”

Together we managed to trace out the contents of several paragraphs, of which the significant parts were as follows: 

I am expecting that my friend Senora Herreria will be in New York by the time you receive this, and should she call on you, I know you will accord her every courtesy.  She has been in Mexico City for a few days, having just returned from Mitla, where she met Professor Northrop.  It is rumored that Professor Northrop has succeeded in smuggling out of the country a very important stone bearing an inscription which, I understand, is of more than ordinary interest.  I do not know anything definite about it, as Senora Herreria is very reticent on the matter, but depend on you to find out if possible and let me know of it.

According to the rumors and the statements of the senora, it seems that Northrop has taken an unfair advantage of the situation down in Oaxaca, and I suppose she and others who know about the inscription feel that it is really the possession of the government.