X-Rays, Apparatus and Methods Page 17

apply these principles in practice, take a trial picture of the hand at 5 inches distance, this being amply sufficient for good definition with such small bones. Say one minute is required for a good result. Then to take a hip joint, we estimate the thickness of the latter to be, say, ten times that of the hand. We must, therefore, expose ten minutes on account of thickness alone. But, owing to the greater distance of bones from plate, the tube must be removed much further to get definition. At least 15 inches from the dry plate should be given — this is three times the distance used for the hand from which the times require to be as the squares of these figures, or as one is to nine. The total exposure must, therefore, be ninety minutes. This method gives something to go upon. After a little experience, perhaps the most convenient thing to do is to arrange a little table, based upon previous results, and showing times and distances required for well-defined results for distinctive parts of the body.

Short Exposures. — We have obtained and do regularly obtain without difficulty well-defined pictures, showing complete detail of the osseous structure involved in times and with distances from dry-plate to reflector, as below:

We have very carefully investigated every claim to quicker exposures than these, often by a personal visit, sometimes by correspondence with personal friends whose reliability was undoubted. We have been unable to learn of any results equally good having been obtained in shorter times by any one at any place with any form of apparatus, and we do not believe any such have been obtained.

Manipulation — Development of the Plate. — Suitable and well-constructed apparatus is not the only essential to good results. A great deal depends upon the operator, and com-