Scientists have developed and built a thumbnail sized microscope that magnifies without lenses.

They said it is one of the biggest innovations since the microscope was invented 500 years ago. They believe it could be mass produced for less than £5 and, powered by sunlight, could help doctors in the developing world diagnose conditions such as malaria, or help in the detection of water-borne parasites.

The "microscopic microscope" has the same magnifying power as a top-quality device, yet is far easier to carry around in the field, said Changhuie Yang, an assistant professor at the California Institute of Technology, where it was built.

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"The whole thing is truly compact, it could be put in a cell phone, and it can use just sunlight for illumination, which makes it very appealing for Third World applications," he said.

"Our research has been motivated by the fact that microscopes have been around since the 16th century, yet their basic design has undergone very little change and has proven prohibitively expensive to miniaturise. Our new design operates on a different principle and allows us to do away with lenses and bulky optical elements."

The new instrument uses computer-chip technology and microfluids, the channelling of fluid flow at incredible small scales.