|MadSci Network: Zoology|
As you probably know the mirror-like membrane in nocturnal animals called the tapetum lucidum reflects light that has already passed through the retina back through the retina a second time, giving the light another chance to strike the light-sensitive rods. Whatever light is not absorbed on this return trip passes out of the eye the same way it came in -- through the pupil. The presence of the tapetum can be observed at night when a pair of glowing eyes reflects back a flashlight or some other light source. This is why these animals see so much better than we do at night because their eye "catches" more photons due to the tapetum layer. Cats, partly due to the tapetum lucidum, can see clearly in just 1/6th the amount of light humans need. Some dogs do not have a tapetum lucidum, but I am not sure about your dog because you didn't fully describe his genetic background. To test your dog properly for a tapetum you need to make sure that the dog is fully "dark- adapted". In other words, the dog needs to be kept in the dark for about 20 minutes BEFORE you shine the bright light in his eyes so that the pupils are fully dilated. If you still get a green reflection from the brown eye and a red one from the blue eye then it could be because the blue eye does not have a tapetum. I am guessing that a red light reflected from the blue eye would mean that this eye is structured similarly to man's - without a tapetum. In man the reflection is red because of the blood vessels in that area. You might also take some flash photos of your dog in a dark room (using a camera WITHOUT red-eye reduction) to see if the pictures that result show different colors reflected from the eyes. If the tapetum is missing in your dog you probably won't be able to tell if he has any deficiencies in his night vision unless you compare him to other dogs of similar breed in a night situation. Comparing your dog's night vision to your own isn't appropriate as humans do not have this feature. If you experiment further with your dog please let me know, I am interested to hear about your results!
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