Sigma ray exposure
Sigma rays were a type of radiation. Continuous exposure made clothing and flesh transparent, revealing the skeleton underneath.
Sigma rays were being evaluated in the 23rd century at Research Satellite-5 as a component of the technology in a sensor scan, to simplify a search for concealed weapons or contraband from a distance. During a demonstration, Montgomery Scott, Spock and James T. Kirk entered an enclosure where they were exposed to the radiation, which revealed their skeletons as if they were viewing animated X-ray images. A room-sized enclosure was used to test the radiation, but a goal of the project was to incorporate sigma rays into a portable scanner. (TOS comic: "A Bomb in Time")
A similar effect of exposing the skeleton was seen briefly when the USS Enterprise traveled through a cosmic storm. The storm disrupted the ship’s shields and allowed a variety of unnamed types of radiation to expose the bridge crew. Cosmic storms could potentially be a source of X-rays, sigma rays, or both. (TOS comic: "Furlough to Fury")
Kirk exposed to radiation
A similar effect was caused by radiation coming from the surface of the adoptive planet of the Sanoora in the year 2268. The name of the radiation was not given, but it behaved like a lower dose of sigma rays, first producing saliva and darkened eye sockets in the subjects, then fading various parts of the skin to expose the skeleton, then also making the skeleton clear. Cumulative exposure proved deadly without repeated injections of an advanced anti-radiation formula. During a visit by the USS Enterprise, Leonard McCoy modified the chemical in such a way that it could neutralize the radiation itself rather than just its effects on the body. Spock then launched a variety of probes to spray the chemical over the surface of the planet. (TOS comic: "The Planet of No Life")