An ocular micrometer is basically a tiny ruler etched into one of the ocular lenses; it can give you a better estimate of the size of a cell, provided you calibrate it with a stage micrometer, which is a microscope slide that has a scale etched into its surface.
The first and advisable thing to do is to Clean up the vomitus carefully using Standard Precautions since there's a high chance of the vomitus to be infectious.
