194 



NA TURE 



[AUGUM II, 1923 



in instructing students in the methods of research will 

 be regarded as forming a part of the teaching. Further, 

 it is stated that research work which enters into the 

 actual preparation of lessons to advanced students will 

 be properly regarded as a duty subsidiary to actual 

 teaching. 



On the whole, then, it may be said that the new 

 circular is more reasonable and more justly favourable 

 than the old. We still feel that teachers in Technical 

 Institutions may be penalised if their full-time service 

 is to be judged by the same standard of actual teaching 

 hours as that which normally obtains in Primary and 

 Secondary Schools. We admit that the circulars do 

 not say they will be so judged, but, on the other hand, 

 there is no statement, direct or implied, that the same 

 standards exactly will not be applied. That would be 

 extremely unfair, and extremely bad educationally, 

 to those teaching subjects which involve experimental 

 demonstration and laboratory preparation, and to 

 those teaching advanced subjects. A definite state- 

 ment on this point would have allayed anxiety on 

 the part of many who are approaching the pension- 

 able age. 



Further, most of the work done by full-time teachers 

 in Technical Institutions is evening work, and this 

 work is necessarily more intensive, and "involves not 

 only a heavier strain in the actual teaching process 

 but also much more complete preparation work than is 

 required for corresponding day work. We regret that 

 this has not been recognised in the new circular, for even 

 though the officers of the Board may recognise it and 

 act upon it, it does not seem fair either to the teachers 

 concerned, or yet to the Principals and Education 

 Authorities, that no definite pronouncement is made 

 on the subject. 



Finally, we cannot help feeling that Circular 1286 

 should not have been issued officially before it had 

 been submitted to representatives of the authorities, 

 governing bodies, and teachers concerned. The Board 

 stated in the first paragraph of that circular that though 

 it contemplated the application of the principles set out, 

 it would be glad to consider any observations by a 

 certain date before arriving at a final decision. The 

 result of the observations is shown by Circular 131 1 ; 

 but surely it would have been very much better had the 

 revision been made before the official issue of the first 

 circular. As it is, there has been much difficulty and 

 misunderstanding, and in some cases these provisional 

 principles have already been acted upon and, in the 

 light of the supplementary and explanatory circular 

 before us, acted upon wrongly. We also feel that it 

 would have been better to have cancelled Circular 1286 

 entirely and to have issued a new one amended on the 

 lines of Circular 131 1. 



NO. 2806, VOL. 1 12] 



Map-making from Air Photographs. 



Generalised Linear Perspective : Treated with Special 

 Reference to Photographic Land Surveying and Militat . 

 Reconnaissance. By J. W. Gordon. Pp. xvi + 184. 

 (London, Bombay and Sydney : Constable and Co., 

 Ltd., 1922.) 215. net. 



DURING recent years much attention has Ijeen 

 paid to air-photography as a means of survey- 

 ing; the present developments of the subject bein^ 

 chiefly due to the varied experience which was gained 

 in the War. The method is still on its trial. There 

 are certain conditions under which it promises to be 

 successful, but no peace-time surveys of any importance 

 have yet been carried out on this system. It is likely 

 to be found of value in flat countries, and for maps on 

 medium scales. Air-photo surveys have been suggested 

 for the mapping of deltas, such as those of the Ganges, 

 the Niger, and the Irrawaddy, and for the surveys of 

 large native towns. The suggestion, made a few years 

 ago, to map a hilly West Indian island in this way, 

 was, probably wisely, " turned down." 



The subject is thus, so far as concerns peace-time 

 surveys, in a tentative stage, and any original con- 

 tribution to the theory is most welcome. Mr. J. W. 

 Gordon has made such a contribution in his book 

 entitled " Generalised Linear Perspective." He gave 

 a demonstration of his methods at the British Museum 

 on March 25 last, and they have been described in 

 popular terms in the Times. His ideas are thus 

 being made well-known. 



The main object of his investigation is to find a 

 direct and simple system of converting an " inclined " 

 air-photograph into a map or plan. In the most 

 general case a photograph is taken in the air, at an 

 unknown height above the ground, of country with 

 unknown undulations and hills ; the camera is tilted 

 at an unknown angle and the direction of the tilt is 

 also unknown. Nowadays, however, thanks to the 

 insistence of the Air Survey Committee, it may be 

 expected that the focal length of the lens will be 

 known in every case, and also the position of the 

 optical centre of the photographic plate. 



The first step in Mr. Gordon's investigation is to 

 choose a horizontal reference plane on which the plan 

 of the ground is to be projected, at a distance from the 

 nodal point of the lens equal to its focal length — a 

 useful simplification, which, however, determines auto- 

 matically the scale of the plan, so that photographs 

 taken at different heights will be plotted on different 

 scales. 



Mr. Gordon introduces us to a new terminology, 

 puzzling at first, and not always very clearly ex- 

 plained, but legitimate. It is necessary' to learn the 



