Related to Apparatus for investigating the role of eye movement in spatial perception illusions.
Description:

Notes on the design of the apparatus:

'It has been suggested to me by students of physics who ought to know that the following simple arrangement ought to present no difficulties.

[sketch of arrangement, with different parts labelled I, D, L, P, L' and C - see also f.3 of Notes on and design for optical stimulation apparatus]

'The light from the lamp I streams through the holes in the disc D, is refracted through lens L, and again through another lens L' in parralel rays on to the plate C. Apparently it does not much matter whether the light from I is thrown on to D in parallel rays or not.- the interruption by means of the pendulum P recurs at the focal point common to both L and L'.

Such lenses as would be required for this purpose need not be at all expensive, and the plate C might be fixed to a stand, and might possibly not require any camera arrangement at all. The ordinary lens which costs a few shillings would not, I inderstand, give an absolutely colourless image - it would be coloured at the fringes. But I understand that slightly more expensive lenses exist which overcome this, after all, rather slight difficulty.

If such an arrangement as the one just sketched us at all practicable, it would really be quite ideal. I just offer you this as a suggestion. It could easily be fixed up I imagine, on the plan, say, of an ordinary camera if necessary.

[sketch of lens and screen set-up here]

The essential difference as I understand [it] is that in the ordinary camera there is no true focal point between the two lenses whereas in the system required there would be. But I may be wrong, and as you must be sufficiently aware by now, thanks both to this morning's conversation and to this letter, I am as ignorant as possible of optics.'