manner to secure and maintain the goggles in position while maintaining a fluid tight interface with his face as is illustrated in fig2 and 4 standard refractive measurements are once again taken to determine the type and power of lens required to enable the individual to clearly see a chart positioned at a standard twenty foot distance it may be necessary to add a powerful auxiliary refracting lens to the refracting device to accomplish this step while making the refractive measurements with the cornea 12 to fluid optical interface it is not necessary to make a second measurement at a perpendicular angle to determine any astigmatic error induced by the cornea since the contact of the fluid with the cornea effectively eliminates the optical influence or power of the cornea since the index of refraction of the transparent corneal material and the index of refraction of the transparent fluid are very nearly the same snell 39 s law indicates that a light ray will not be bent or refracted as it passes from the transparent fluid through the cornea and into the aqueous humor since the index of refraction of these three materials is virtually identical the light ray will be refracted by crystaline lens 14 but this effect will be subtracted out in the mathematical equations to be discussed below finally the distance between refracting device 10 and the surface of cornea 12 and the distance between the surface of cornea 12 and the retina must be measured this first distance can be determined by using a standard measuring device between the plane of refracting device 10 and the anterior surface of cornea 12 the distance between the anterior surface of the.