Fossils in shales can be difficult to see and photograph, because they are pressed flat and often do not differ much in colour or tone from the surrounding rock. Simple methods are available, however, that can improve the photographs dramatically. With the help of polarizing filters, such as are found in some sunglasses, directly reflected light can be filtered out. Skin doctors use this method to see and photograph injuries deep in the skin (such as acne), but it is also excellent for fossils that consist of shiny films in shale. Images of specimens from the famous Burgess Shale (containing preserved soft bodies of various animals from the Cambrian Period) show stunning results when processed by this method. The method is also very useful for fossils that are too delicate to be immersed in a liquid (otherwise a common method to reduce glare in photographs). Pictures taken with and without polarized light can also be combined to bring out structures that are invisible in the separate images. This can be done with traditional film techniques, but they are much more efficient when applied to digital images using an image-editing program. The same method can be used to bring out colour differences in a fossil.