We propose a laboratory experiment on the quantitative study of the normal dispersion of light. A triangular isosceles prism made of flint glass TF3 is used as the object of study, and we describe a simple and affordable device for observing and photographing the dispersion spectrum on a smartphone. A possibility of the quantitative investigation of light dispersion is provided by using a modern miniature mercury lamp emitting a line spectrum of mercury vapour against the background of a solid spectrum of white light. The method of processing experimental results is stated.