The ultimate goal of our research is to measure in vivo the RI of a superficial area of a cornified layer. However, since this is difficult, most studies have performed ex vivo measurements of slices of skin.10–12 The main source of difficulty of in vivo measurements is undulations in the skin surface.16 RI measurements generally require precise measurement of the reflectance as a function of the incident angle at the interface between a prism and a sample. Consequently, a flat sample surface is needed to ensure good contact between the prism and the sample. Hence, some modification is required to apply this method to samples with rough surfaces. In some studies, the sample was pressed against the prism with great pressure so as to achieve good contact.11,12 However, this method is not suitable for in vivo measurements. Another significant problem of in vivo measurements is their long acquisition time, which is inconvenient for the subject. Furthermore, skin conditions will change over time since the water content varies due to occlusions. Due to these difficulties, almost all previous studies performed ex vivo measurements of skin slices. but doing this causes other problems. First, this method is destructive. In addition, after being cut off, skin will dry out since its water supply is disrupted, and that changes its RI.1,17 To make the conditions of samples close to those of living skin and to increase the contact area, other studies have used saline10–12 or high-RI immersion oil.17 However, various kinds and amounts of liquid are applied, and these factors will affect the RI. Therefore, in vivo measurements are desirable for evaluating the RI under different conditions and for different individuals.