Animal Eyes by Michael F. Land

Author:Michael F. Land
Language: eng
Format: epub
Publisher: Oxford University Press
Published: 2012-08-24T04:00:00+00:00

The stepped-mirror eyes of Dolichopteryx

In Chapter 4 we discussed how some deep-sea fish have evolved double eyes, with a ‘normal’ eye looking towards the surface and a secondary eye pointing downwards to detect luminescent objects in the dark waters below. Recently a very unusual example of a secondary eye has been found in which the optical system is not a lens or lens-pad (see Chapter 4, Fig. 4.11), but a concave mirror of unique design (Wagner et al. 2009). Dolichopteryx longipes is a rarely encountered mesopelagic fish with double eyes, the larger of each pair directed upwards and the smaller downwards (Fig. 6.5a and b). The secondary eye is a diverticulum of the larger, but in contrast to the main eye it forms an image using a mirror. Unlike the scallop argentea, this is not simply a ‘front-silvered’ mirror, but it is made up of distinct stacks of reflecting platelets, probably composed of guanine crystals. These platelets make angles to the substrate membrane that increase in a regular manner from top to bottom of the structure (Fig. 6.5c).

It is not easy to produce a mirror that gives a good image over a wide angle (in this case about 48°). A spherical surface, unless corrected with a lens, has severe spherical aberration. A parabolic surface gives an excellent image for a point on its axis, but the image quality deteriorates very rapidly away from that point. No single surface can do the job. By producing a stepped surface the Dolichopteryx mirror has largely solved the problem.

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.