Abstract: In the experiment of measuring the refractive index of a prism with a

spectrophotometer，a key step is to adjust the object stage so that its main axis is perpendicular

to the optical axes of the telescope and the collima- tor，which is called the standard case. It is

analyzed that the influence on the measurement of apex angle and mini- mum deviation angle

theoretically and experimentally when there is a deviation of the object stage. The experiments are

performed for two cases. One case is that，in the telescope，it can be found that a green

cross image reflectes from at least one surface of the double face plane mirror. In this case，the

deviation is marked by recording the posi- tion of the green cross image. The other case is that

the deviation of the object stage is measured with an extra sys- tem. In this case，the deviation

degree of the object stage can be recorded directly. In experiment，the variation of the apex angle

and minimum deviation angle of the prism with the deviation of the object stage is measured

in the two cases. And the object stage’s deviation range，at which the experimental

results has smaller deviations than standard measurement errors，is determined.

Key words: spectrometer, triangular prism, apex angle, minimum deviation angle