Complex sentence: With gaseous and solid samples, Raman scattering can still be too weak to easily detect, and so for these samples, the sample holder is placed between two mirrors that continuously pass the laser light back and forth through the sample.
Split into simpler sentences: With gaseous and solid samples, Raman scattering may still be too weak to easily detect. For these samples, the sample holder is placed between two mirrors that continuously pass the laser light back and forth through the sample.