460 JOHN W. G O W E N 



radiation occurs in a series of peaks, K, L, M, etc., which correspond 

 to the various electronic energy levels in the atom. For copper the K 

 peak is at 1.5 A. ; for silver, at 0.49 A. 



Interposing a sheet of metal between the object and the X-raj^ 

 tube has the effect of filtering out the radiations of the longer wave- 

 lengths, but it also results in the emission of secondary X rays which 

 are characteristic of the atomic number of the metal used as a filter. 

 These secondary X rays are always longer in wavelength than the 

 original X-ray beam. In case they are absorbed by a filter of still 

 lower atomic number, perhaps aluminum, characteristic X rays of 

 aluminum are emitted, but of course are still longer in wavelength. 



The significance of these facts to the investigatoi' comes in empha- 

 sizing the experimental arrangement that must be made for critical 

 experiments and the care with which the measiu'ements of the X rays 

 received by the specimen must be taken. 



It is not always possible to meet these conditions exactly. An 

 investigator interested in irradiating Drosophila sperm and then utiliz- 

 ing them for fertilizing Drosophila eggs requires sperm irradiated in 

 the intact adult male fly. X radiation incident to the fly can be esti- 

 mated in r. units, but absorption in the chitinous body wall that sur- 

 roimds the abdomen of the fly and back-scatter of the tissues to the 

 sperm become diflScult to measure. The dosage measurements could 

 be made from two positions, i.e., at the body surface before any X 

 radiation has passed through the fly's tissue and after the radiation 

 has passed through the fly's body. The difference between these 

 measurements shows the amount of radiation absorbed in the fly's 

 tissues. Neither of these measurements is what the investigator 

 wants. He wishes to knoM^ the radiation incident to the sperm. Two 

 assumptions might be made to facilitate this estimate : that the sperm 

 are in some definite position within the body (as at its center) and 

 that the laj^ers of tissue above and below the two surfaces are absorb- 

 ing uniformly and with negligible back-scatter. X-radiation ab- 

 sorption proceeds exponentially as follows: 



/ = he 



-fix 



where /o is the intensity of the radiation incident to the fly, and / is 

 the intensity that passes through the fl.y; x is the thickness of the fly 

 and ju is an "average" absorption coefficient of the fl}^ Then Ic = 

 /oe"""^^^ — /o(///o)'^', where Ic is the intensity at the center of the 



fly. 



