The scheme for taking a pair of photographs of opposite sensitivity for each flow is a useful one in certain problems, but is unsuitable for use in a routine experiment (in which several hundred photographs may be required) because of the additional labour and expense involved. This is particularly so because, apart from resolving any ambiguity which may occur in regions of intense density gradient, the second photograph usually will not give much additional information about the flow. Moreover, the scheme does not overcome the other difficulties associated with the use of an apparatus having inadequate range. The most serious of these is that the apparatus is unable to differentiate between the strengths of density gradients when they exceed the value which deflects the image of the light source completely on to, or away from, the knife edge. Photographs illustrating this effect are reproduced in Ref. 1; the use of inadequate range (i.e. too much sensitivity) is a common fault in wind-tunnel photographs.