Cobalt disc: chromatic test
 Catalogue Number: 947 Cobalt disc: chromatic test Category: Equipment Sub-Category: Equipment, other clinical Time Period: 1900 to 1939 Description Of Item: Cobalt blue glass in trial case rim. Diam: 3.8 cm Historical Significance: The cobalt disc was once a standard trial case accessory but is now obsolete. It is a simple subjective test based on the chromatic aberration of the eye. The lens material absorbs most of the middle region of the visible spectrum while transmitting some from each of the red and blue ends. A small white light source viewed through the lens will form overlapping red and blue diffusion circles, the appearance of which will vary depending on the refractive error of the viewer. Refer; The Optician's Manual, C H Brown, The Keystone Press, Vol 1, p285, Cat No 815 How Acquired: Donated by Brian Flynn, member of the college and former honorary archivist of the Kett Museum Condition: Very good Location: Archive office. East wall shelves unit 3. Drawer 1 |