115 
to an improvement in geographical maps, and is, therefore, of value to all. As no one wishes to 
speak further on the subject of testing methods, the Chairman calls on Prof. Jackson to make a 
statement which was not on the programme. 
Prof. K. B. Jackson: Recent work at Toronto University has been directed K. B. Jackson 
towards the determination of optimum photographic and observational tech 
niques for the maximum interpretability of aerial photographs. 
Under the former (photographic techniques) the revival of the method of un 
sharp masking was proposed in 1950—and greatly improved by Mr. Eden of the 
Colonial Surveys, England. Since then the scanning-spot printer has come into 
existence in several forms and it is freely admitted that when the scanning spot 
can be reduced in size the results will approach those obtainable by unsharp 
masking, and that the method is to be preferred. 
These methods of printing, with accurate contrast control, permit the use of 
high contrast positive material, whereby an improvement in resolution in the 
positive is available. 
It now becomes necessary to consider the treatment of the negative (exposure 
and development) which will give the best resolution, bearing in mind the new 
possibilities of drastically increasing the local contrast in the positive by one or 
other of the new methods. 
To this end a series of experiments have been carried out in Toronto to de 
termine the effect on resolution, in the film alone, of variations in exposure and 
development. This has been done elsewhere with minor differences in procedure 
and method of displaying the results, but it may be of interest to report the 
results obtained at Toronto. 
Contact prints of a low contrast (.14) resolution target (made by Dr. How- 
lett’s laboratory N.R.C. Canada) w^ere made through a stepped wedge on Kodak 
Super XX aerographic film with a uniformly bright surface source at 8 times 
its diameter from the printing surface—thus simulating the illumination from 
a camera lens operating at an aperture of f/8, but eliminating the effects of lens 
resolution, or the lack of it. 
The test strips were developed in a constant manner except for time of de 
velopment. 
The densities of line and background tone areas were measured and the mean 
density was plotted against log exposure for each wedge step. 
The resolution in lines per millimeter was read through a 20Xbinocular micro 
scope, 3 targets per wedge step, and the target numbers plotted on each D-log 
E curve. 
Contours of equal resolution were then drawn over the family of D-log E 
6,— 
curves with a contour ratio of 1 : \ 2 lines per mm. In a second group density 
difference is plotted against mean density with contours of equal resolution 
superimposed. 
The results indicate a maximum resolution (exceeding 30 1/mm) at a mean 
density of 0.3 which is twice as great as that obtained at a density of 2.0. 
It would therefore appear desirable when special printing methods are used 
to avoid densities in the negatives above, say .5, by reducing the gamma to 
which the film is developed and increasing as far as practicable the time of ex 
posure. 
Similar tests are to be carried out on slower finer grain emulsions to determine 
the optimum combination of film, exposure, development to give maximum re 
solution in the negative assuming that the necessary local contrast can be ampli 
fied by printing with the new methods on high contrast positive material. 
Le Prof. K. B. Jackson indique l’intérêt qu’ont les méthodes de tirage par compensation au 
moyen d’un masque flou ou au moyen d’un tube de télévision à spot modulé. Ces méthodes 
permettent d’utiliser des émulsions positives très contrastées. Il en résulte que l’on doit reconsidérer 
le traitement du négatif: temps de pose et conditions de développement. Il décrit les expériences 
qu’il a conduites dans ce sens à Toronto. Elles ont donné une résolution deux fois meilleure pour