Reflecting Body | Impression by Transmitted Rays. | Impression by Reflected Rays. |
|---|---|---|
| Brass | Strong | Fairly good |
| Toolsteel | Barely perceptible | Very feeble |
| Zinc | None | Very strong |
| Aluminum | Very strong | None |
| Copper | None | Fairly strong but much less than zinc |
| Lead | None | Very strong but a little weaker than zinc |
| Silver | Strong, a thin plate being used | Weaker than copper |
| Tin | None | Very strong about like lead |
| Nickel | None | About like copper |
| Lead-glass | Very strong | Feeble |
| Mica | Very strong | Very strong about like lead |
| Ebonite | Strong | About like copper. |
By comparing, as in previous experiments, the intensity of the photographic impression by reflected rays with an equivalent impression due to a direct exposure of the same bulb and at the same distance, that is, by calculations from the times of exposure under assumption that the action upon the plate was proportionate to the time, the following approximate results were obtained:
Reflecting Body | Impression by Direct Action | Impression by Reflected Rays. |
|---|---|---|
| Brass | 100 | 2 |
| Tool steel | 100 | 0.5 |
| Zinc | 100 | 3 |
| Aluminum | 100 | 0 |
| Copper | 100 | 2 |
| Lead | 100 | 2.5 |
| Silver | 100 | 1.75 |
| Tin | 100 | 2.5 |
| Nickel | 100 | 2 |
| Lead-glass | 100 | 1 |
| Mica | 100 | 2.5 |
| Ebonite | 100 | 2 |
He stated that while these figures can be but rough approximations, there is, nevertheless a fair probability that they are correct, in so far as the relative values of the sciagraphic impressions of the various objects by reflected rays are concerned.
In order to devise means for testing the comparative reflecting power in a more decided manner, he laid pieces of different metals side by side upon a lead plate. Consequently the reflecting surface was formed of two parts corresponding to the two metals. [§ 80]. The vertically perpendicular partition of lead served to prevent the mingling of the rays from the two metals. Ingenious precautions were taken; as for example, so arranging matters that upon equal areas of the two plates, equal amounts of X-rays impinged. [§ 80]. He undertook to determine the position of iron in the series by thus comparing it with copper. It was impossible to be sure which metal reflected better. The same regarding tin and lead and also in reference to magnesium and zinc. Here, a difference was noticed, namely that the magnesium was a better reflector.
He has made practical application of the power of the substances to reflect a certain per cent. of the rays by employing reflectors for the purpose of reducing the time required for exposure of the photographic plates. It admits, he stated, of the use of reflectors in combination with a whole set of discharge tubes, whereby rays which would be otherwise scattered in all directions are brought more nearly to a single direction of propagation.
From Sciagraph of Knee-joint. Straight, Front View.
By Prof. Goodspeed. Photo. Times, July, ’96.
It might be argued, that in as much as zinc would reflect only about three per cent. of the incident rays, no practical gain would ensue in sciagraphy by the use of a reflector. He pointed out the falsity of such an argument. In the first place, the angle employed in these tests was 45°. With greater angles, the proportion of reflected rays would be greater assuming that the law of reflection is the same as that of light. By mathematical calculation and tests, he showed that there was no doubt whatever about the advantage of using reflectors. He obtained a sciagraph, on a single plate, of the ribs, arms and shoulder, clearly represented. He stated the details as follows. “A funnel shaped zinc reflector two feet high, with an opening of five inches at the bottom and 23 inches at the top, was used in the experiment. A tube similar in every respect to those previously described, was suspended in the funnel, so that only the static screen of the tube was above the former. The exact distance from the electrode to the sensitive plate was four and one-half feet.”
147. Discharge Tube Placed in Oil.—When the E. M. F. was increased, by having the discharge tube, as usual, in open air, sparks formed behind the electrode, and within the vacuum, and endangered the life of the discharge tube. He obviated this difficulty partly by having the electrode located well within the evacuated space, so that the wire leading to it was unusually long. By excessive E. M. F., also, streamers broke out at the end of the tube. To overcome all difficulties in connection with sparking and breaking of the tube, he followed the proposition of Prof. Trowbridge, and submerged the discharge tube in oil, [§ 11], at end, and [§ 13], which was continually renewed by flowing into and out of the vessel in which the discharge tube was contained, all as shown in the accompanying figure, p. [157], “Discharge Tube Immersed in Oil.” The discharge tube, t, may be recognized by its shape, and it is located horizontally in a cylindrical tube lying sidewise upon a table. To regulate the flow of the oil, the reservoir may be raised and lowered by a bracket, s. The X-rays enter the outside atmosphere by passing first through glass, then oil, and then through a diaphragm of “pergament” forming the right hand end of the oil vessel. When the results were compared with those obtained by Roentgen in his first experiments, the rays were found so powerful that it is not surprising that Tesla was able to obtain more definitely a closer knowledge of the properties of the rays. Roentgen obtained, with his tube and a screen of barium platino cyanide, a shadow picture of the bones of the hand at a distance of less than 7 ft., while Tesla obtained a similar picture with a screen of calcic tungstate, and with his tube immersed in oil at a distance of 45 ft. Tesla also made sciagraphs with but a few minutes’ exposure at a distance of 40 ft., by the help of Prof. Henry’s method, i.e., with the assistance of a fluorescent powder. [§ 151]. He referred also to Salvioni’s suggestion of a fluorescent emulsion. He attributed to Mr. E. R. Hewitt the conjecture that the sharpness of the sciagraphs might be increased by a thin aluminum sheet having parallel groves. Several experiments were made, therefore, with wire gauze, as well as with a screen formed of a mixture of fluorescent and iron-fluorescent powders. With the strong power of the rays as obtained by Tesla in combination with such adjuncts, the shadows were sharper, although the radiation, of course, was weakened by the obstruction. [§ 107b].