[cox A callbndarI experiments ON THE X-RAYS 177 



extremities were taken, as well as photographs of various parts of the 

 healthy body including the skull and trunk. 



Exposure and Development. — In taking photographs through the 

 trunk, Ave found that little or nothing was gained by prolonging the 

 exposure bej'ond ten minutes or a quarter of an hour. A longer expo- 

 sure appeared to give a Hat over-exposed result, in which the fainter dilter- 

 ences of transparency were, to a great extent, obliterated. For the same 

 reason we found it best to use a fairly strong develoj^er, in order to 

 strengthen the conti-ast as much as possible. We generally used rodinal 

 of strength 1/10, and continued the development for ten minutes on the 

 average. In many eases the negatives were subsequently intensitied with 

 bichloride of mercury in order to heighten the effect. In some cases this 

 method was found to bring out slight differences of density which were 

 otherwise too faint to be appreciated. 



The X-ray photograph differs from that produced by ordinary light, 

 in that the action of the X-rays extends through the whole thickness of 

 the film, whereas that of ordinary light is confined to the surface. The 

 photographic film is very opaque to the actinic rays of the spectrum, but 

 is very transparent to the X-ra3'S, which are capable of penetrating 

 many successive films without apparent weakening. It is therefore 

 necessarj^ to continue the development until the action has oxteuded 

 through the whole thickness of the film. It is also evident that greater 

 density may be obtained by using thick films, and that the time of expo- 

 sure might be enormousl}' reduced if it were possible to discover a sensi- 

 tive film capable of absorbing the w^hole energy of the X-radiation in a 

 single thickness. Some advertisers claim to have reduced the time to 

 less than one-hundredth in this manner, but so far as we can discover 

 their results do not appear to be in any way superior to those which we 

 have obtained with ordinary plates. 



Stereoscopic X-Ray Fhotographs. — In locating a small object in the 

 thicker parts of the body, it is often necessary to know the depth at 

 which it is situated. Various more or less complicated methods have 

 been proposed for accomplishing this. The majority of the proposed 

 methods turn on securing a pair of photographs either taken in ditterent 

 directions, so that the coordinates of the object may be deduced, or else 

 taken from slightly different points of view, so that they may be com- 

 bined into a single stereoscopic picture. 



We have found that the same result ma}^ be moi-e simply and accu- 

 ratel}' attained in a single photograph in the following manner. A pho- 

 tograph is taken in the ordinary way but with a rather shorter exposure 

 than usual. The tube is then moved through a carefully measured 

 distance, generally one or two inches, and another exposui-e is taken on 

 the same plate without moving either the plate or the patient. The 

 distance of the focus point from the plate is also measured. On 



Sec. III., 1896. 11. 



