Pulp Fibers and Paper 



189 



main disadvantages of reflection microscopy as used 

 by us at present. Much of the image is lost either in 

 shadow or in ground hidden from view, a situation 

 that is aggravated when rougher surfaces are 

 studied. This difficuUy and the disadvantage of a 

 severely foreshortened image could be overcome by 

 the use of higher angles of illumination and viewing. 

 Two factors however have limited our use of the 

 reflection method to low angles. In the first place, 

 the greater the angle of deviation of the beam the 

 poorer the resolution. This appears to be due to the 

 greater spread of velocities of electrons scattered at 

 high angles. It has been shown (7) that this velocity 

 spread is due to the effect of contamination and that 

 provided that the contamination is suppressed by 

 subjecting the specimen to ionic bombardment while 

 the electron beam impinges on it, quite a high reso- 

 lution can be obtained when metal specimens are 

 examined using angles of deviation as high as 25 \ 

 Secondly we were restricted in our early work to 

 relatively low angles of deviation by the rapid fall 

 ofT of intensity of the scattered beam at high angles. 

 The increased intensity of the incident beam required 

 to meet this aggravated the problem of beam dam- 

 age. The angle of deviation of 17 that we finally 

 adopted (and which appreciably exceeds that gener- 

 ally used) was a compromise between the conflicting 

 requirements of high resolution and minimum beam 

 damage on the one hand and small shadow areas 

 and a low foreshortening on the other. However, 

 with the advent of inert solid replicas that are 

 capable of withstanding high beam intensities, it 

 appeared worthwhile investigating whether a useful 

 image could be obtained at high angles even though 

 we have no facilities at the moment for ion bombard- 

 ment. With an angle of deviation of 28 ' the image 

 obtained was rather poor. This may be due in part 

 to the excessive chromatic aberration when the objec- 

 tive lens is operated at long focal length, a necessary 

 condition for high angle reflection work with the 

 Met-Vick EM 3 microscope without modifying it 

 considerably. Further work is needed to determine 

 whether a resolution acceptable for our purpose can 

 be obtained by modifying the microscope so that the 

 focal length can be reduced or whether the resolu- 

 tion is severely limited by the effect of contamination. 

 It remains to be seen whether the scanning electron 

 microscope (9, II) will give a satisfactory solution. 

 It is now possible to assess the advantages and 

 disadvantages of the reflection method more clearly. 

 We believe the method is of some value in this 

 field if used as a method complementary to other 

 techniques of microscopy. It has been shown that the 



plastic matrix from which the metal solid replicas 

 are made can be used indestructibly both for light 

 microscopy (6), and for the production of carbon 

 replicas for transmission electron microscopy (10). 

 It is therefore possible to obtain micrographs of the 

 same ret^ion of an individual fibre as follows: — 

 (a) a light micrograph showing a large area of 



fibre but with limited resolution; 

 (/)) a reflection electron micrograph with its 

 oblique viewpoint and remarkable three di- 

 mensional appearance; and 

 ((■) transmission electron micrographs of selected 

 areas at a magnification sufficient to resolve 

 ccllulosic microfibrils (100 200 A thick). 

 When three such images are considered the com- 

 plementary information from each should enable 

 the finest structural detail to be related to the fibre as 

 a whole in a way not previously possible. 



The chief drawbacks of the reflection method as 

 described here then are the severe foreshortening 

 and the loss of information in shadow and "dead 

 ground"". The reflection method does not permit 

 angular or linear measurements to be made accu- 

 rately with the exception of heights which can some- 

 times be determined with a fair degree of precision. 

 On the other hand when used in conjunction with 

 complementary methods of microscopy the three- 

 dimensional aspect of its image, its sensitivity to 

 height, and its great depth of field which permits 

 montages of considerable lengths of a fibre to be 

 made economically are all of value in revealing the 

 external form of the specimen. 



References 



1. Amboss, K., Emerton, H. W., and Watts, J., Proceed- 



ings of the International Conference on Electron 

 Microscopy, London 1954, Publ. by Roy. Micro- 

 scop. Soc, London. 560-564 (1956). 



2. Bradley, D. E., Brit. J. Appl. Phys. 6. 191-195 (1955). 



3. Chapman, J. A. and Menter, J. W., Proc. Roy. Soc. A 



116, 400-407 (1954). 



4. Emerton, H. W., /. Roy. Micro.scop. Soc, Ser. 3, 74, 35- 



41 (1954). 



5. — Re.searcli {LoiuL), 7, S56 (1954). 



6. Emerton, H. W., Page, D. H., and Watts, J., Brit. 



Paper and Board Maker.s' Assoc. Proc. Tech. Sect. 37, 

 105-125 (1956). 



7. Fert, C, Marii, B.. and Saporte, R., Compt. rend. 



accul. sci. (Paris) 240, 1975-1978 (1955). 



8. Menter, J. W., /. Inst. Metals 81, 163-167 (1952). 



9. McMui lEN, D., Proc. Inst. Elec. En^rs II 100. 245-259 



(1953). 

 10. Page, D. H., These Proceedings, page 285. 

 I 1. Smieh, K. C. a. and Oateey, C. W., Brit. J. Appl. Phys. 



6, 391-399 (1955). 



19 - 568204 Electron Microscopy 



