DORMANT IMPRESSIONS ON THE RETINA. 435 



3rd. Levelling. If the glasses thus prepared have not the 

 meniscus of cement of uniform thickness for the same dis- 

 tances from the centre (a defect ascertainable in a moment by 

 looking through the telescope at a real or artificial star), warm 

 them again until the cement is melted, and no more, and with 

 the two hands gently pinch the edges together, turning the 

 glasses constantly round until cold, or nearly so. I have never 

 found this fail, if neatly executed. 



If air-bubbles appear in the cement, slide the one glass 

 when hot over the other until the bubbles escape, and then 

 gently slide it back. 



Many details can only be learned by practice. I have 

 given all the points as to materials and manipulation which 

 occur to me as essential. I strongly urged on Mr. Cooke some 

 years since to take up the subject, as I am, of course, not suf- 

 ficiently a practical optician to make good telescopes. I hope 

 it will now be taken up in earnest. 



ON A MODE OF REVIVING DORMANT IMPRESSIONS 

 ON THE RETINA. 



Philosophical Magazine ', May 4, 1852. 



FIRST. Look steadily at a luminous object, sufficiently bright 

 to be borne by the eyes without great inconvenience, then 

 turn the eyes upon a dark body or dark space : an image of 

 the object previously looked at will be seen, a fact familiar to. 

 everybody. When the image has completely faded away and 

 is no longer visible, pass backwards and forwards between the 

 eye and the dark body a white substance, say a sheet of paper : 

 the image will be immediately revived, and may be thus in- 

 definitely reproduced. 



If the light is, in the first instance, not sufficiently vivid to 

 produce the continued impression on the retina, but is nearly 

 so, the invisible image may be brought out, or first rendered 

 visible, by moving the white object between the eye and the 

 dark body or dark space looked at. The white substance 

 should be in a situation exposed to light, so that its whiteness 

 affects the eye, and not held in shadow. After a little prac- 



FF 2 



