effects equally to be depended on, and equally available in demonstration.
343.
As it is thus merely necessary that the image should be coloured, so the large water-prism before alluded to affords us the best means of effecting this. A pasteboard screen may be contrived to slide before the large surfaces of the prism, through which, in the first instance, the light passes uncoloured. In this screen openings of various forms may be cut, in order to produce different images, and consequently different accessory images. This being done, we need only fix coloured glasses before the openings, in order to observe what effect refraction produces on coloured images in an objective sense.
344.
A series of glasses may be prepared in a mode similar to that before described (284);