Final Answer:
On an Abbe retometer, significant figures should be read:
d) Varies by instrument precision
Step-by-step explanation:
The number of significant figures to be read on an Abbe retometer depends on the precision of the instrument, hence option d. Unlike some measurements where a fixed number of significant figures are standard, the Abbe retometer, being a device used for measuring the refractive index of transparent materials, is influenced by the instrument's inherent precision, as mentioned in option d.
If the Abbe retometer is calibrated to a high degree of precision (option d), it may allow for the measurement of more significant figures. On the other hand, if the instrument has lower precision (option d), it may be limited to providing fewer significant figures.
This variability is crucial as it reflects the instrument's capability to discern subtle changes in the refractive index, and the user must consider this when interpreting measurements. Therefore, the correct answer to how many significant figures should be read on an Abbe retometer is that it varies by instrument precision, corresponding to option d.