X-RAY AllCROSCOPY 



Fig. 2. Magnified radiograph of lead (A) and 

 aluminum (B), both about 1 mm thick. Radiograph 



o 



(1) is made with soft x-rays about 1.8 A, radio- 



o 



graph (2) with 0.5 A. The specific x-ray absorption 

 is not conspicuous in the radiograph made with 

 soft x-ravs. 



literature that some new or smaller details 

 may be resolved with x-rays obtained at 

 tensions lower than 3 kv. With 2.0 to 4.0 A 

 the following microstructures can be resolved 

 as described by different authors: cell, i.e., 

 cytoplasm, nucleus (sometimes even nucleo- 

 lus), cell membrane, sometimes cytoplasm 

 inclusions, fibers, some blood elements, etc. 

 In other words, all structures can be resolved 

 which may be seen within the limits of mag- 

 nification of a micro radiograph (see below). 

 The resolution of such structures as chromo- 

 somes, Golgi apparatus, etc. depends more 

 on the resolving power of the emulsion than 

 on the qualitative and geometrical resolu- 

 tions. 



Therefore the tendency of some authors 

 to use ultra-soft x-rays for every kind of 

 microradiography does not appear sound 

 from the point of view of micromorphology. 



The beryllium window of the Machlett 

 tube AEG-50A is about 1 mm thick. Ac- 

 cording to Lurie (86) (see Table 1) this win- 

 dow reduces by half the intensity of a beam 

 of x-rays with the effective wavelength 



o 



around 2.0 A. It is therefore possible to get 



o 



2.0 A x-rays out of this tube. However, 

 mindful of the great absorption of these 



rays by air (approximately 20 times more 



o 



than of 1 A rays) (110) one has to use a 

 \ery short distance between the beryllium 

 and the object-emulsion. At a longer distance 

 the air between window and object has to be 

 evacuated. X-rays with the wavelength 



o 



longer than 2.5 A are absorbed by 1 mm 



o 



beryllium. According to Lurie, x-rays 4.2 A 

 long are absorbed by half, even by a beryl- 

 lium plate as thin as 0.14 mm, which thin- 

 ness can be achieved only under laboratory 

 conditions. The industrial limit is now 1 mm. 

 As was mentioned above, the air absorption 



o 



of x-rays 4.2 A and longer is very consider- 

 able. For example, according to Victoreen 



o 



(110), this absorption for rays 4.2 A long is 



o 



almost 70 times more than those of 1 A. Evi- 

 dently, all microradiographs with rays longer 



o 



than 2.5 A must be done in vacuum and the 

 tube window, if any, has to be very thin and 

 made of metal which occupies a low place 

 in the atomic chart. Aluminum of 50 n thick- 

 ness is used for this purpose by most workers. 

 Lamarque pointed out that he used lithium 

 but this metal, although a poor absorber 

 of x-rays, is very difficult to handle. Linde- 

 mann glass (containing boron, lithium and 

 beryllium instead of silicon, sodiiun and 

 calcium of ordinary glass) is used in Europe 

 for windows. This glass is hard and resistant 

 to atmospheric pressure even in thin sections. 

 However, the x-ray absorption by Linde- 



Table 1 



594 



