Overall, it is cheap, easy, and quick to implement the distributed backbone network. Collapsed backbone. This type of backbone uses single, powerful router as the central connection point for multiple subnetworks. As Figure 4 shows, the central device is the highest level of the backbone.arallel backbone. Parallel backbone is a variation of the collapsed backbone, where devices are having more than one connection between them. As Figure 5 shows, there are multiple connections between the high level routers and the network segments.
The most common method to measure visual acuity is the Snellen eye chart. Like all eye charts, the Snellen chart tests a patient's ability to resolve high-contrast letters and is satisfactory if their vision is good. Unfortunately, the chart does not have enough sensitivity for patients with poor vision.
photostress test A test to differentiate the cause of a reduced visual acuity in one eye, between a lesion in the optic nerve and a disease in the fundus of the eye. A bright light is directed into the eye with the best acuity, for 10 seconds, while the defective eye is covered.