While Photometry at low intensities is a difficult procedure at best, if the light varies in intensity or is a flash, accurate measurements become well-nigh impossible.
"The Nature of Animal Light"
E. Newton Harvey
From the point of view of observation, improved methods in the use of the spectroscope and increase of accuracy in Photometry will certainly lead to a great increase in our knowledge within the next few years.
"Darwin and Modern Science"
A.C. Seward and Others